I think it unlikely that what you propose would work. Here's why: propane-fueled engines run on LIQUID propane. They have a carburetor (similar to a gasoline engine) that is kept warm enough that it vaporizes the liquid propane before the vapor enters the cylinders. The liquid-to-gas expansion has a cooling effect.
Now a gas grill regulator only regulates propane GAS from bottle pressure down to a very low pressure. A fish cooker regulator still works with the gas but allows for a higher pressure.
I believe that there will not be enough gas volume provided to run the engine.

Thanks for that info. It does have a carb. A forklift uses a similar set up to my engine, but somewhere along the line, the carb is going to be cold until the engine warms up. Do you/anybody have any idea what type of regulator I would need, possibly a part number?

I have manuals for a later Onan twin that includes conversion to propane -- I'll see if there's information that'll help you. I might also be able to suggest a few places to post it for sale, depending on specs.

Model NH-MS/2305C
SERIAL: 0172356016

The NHMS with a C at the end of the model number was made for a propane generator and should have a tapered crankshaft. (NH= engine model. M= marquis genset, S= fuel type propane, 2305= unit model, C= genset). the The onan marquis gen set is a 1800rpm unit 14hp@1800rpm 60 ci displacement.

Are you sure it runs on liquid propane? The tank may hold liquid propane under pressure, but the liquid vaporizes in the tank and then flows out through the regulator as a gas. You'd get extreme amounts of icing if the liquid to gas conversion happend in the carb, regulator or anywhere else outside the tank. Also, the liquid expands 270% when changing to gas. I doubt you'd even need a spark or combustion if you were pumping true liquid propane into the combustion chamber. The expansion alone would provide your power.

The regulator for a gas grill should work.

For a test, you could probably just point the UNLIT end of a propane torch into the carb inlet. The engine should run. It won't run well because you are manually metering the gas supply, but it will run.

That's a very interesting idea about the torch fuel. I could try that. Somewhere out there is a person who has one of these and would know for sure what parts to use, if it were put into service.

Here is a link that might be useful: {{gwi:298801}}

I spent all of the afternoon working on this engine, trying to get it to run. It tried to run, but never caught on. I checked the spark and after much testing, determined that the coil is bad. One side puts out spark and the other is weak to no spark. It's a two lead coil and I'm going to take an identical one of my John Deere 316 and try to run it that way. By the way, putting propane gas into the carb seems to work, but I don't have both plugs sparking yet.

Tried again with a good coil. No go. Compression is 80, probably a bit low. Opinions? or is it the type of fuel.....propane torch?

My apologies. I couldn't figure out where I'd seen this thread.
Not that I can help. All I could find is a parts breakdown of an LPG converter kit for a P224G. FWIW, the kit contains two converters -- things that look like they convert liquid to gas.
Hope you get it running.

I priced an Onan regulator set-up and it was $750+

At this point, I'm considering a gasoline carb set-up, or just trying to relieve myself of this, otherwise excellent, engine, as is.

I wonder if you could sell it on an RV forum or get a sales lead from one of the local RV dealers. Somebody has to want it.
I also wonder if there are differences in compression ratio and valve timing for a propane engine and whether converting it to gasoline would be reasonable.
There's a guy on Yoohah's Gravely Club that has been messing with the gasoline NH's for some time and has a parts engine posted there now -- I believe a CCK, which might have what you need to do a conversion. He might know something about the propane versions, or his parts engine might be what you need, or he might be interested in yours. He goes by bigpretty12. Unfortunately, he's in Oklahoma, far from the Lehigh Valley (where I live too).
New Onan parts are brutally expensive, and figuring out what will work can be impossible. There are always a bunch on eBay, but who knows what.
Good luck

Thanks for the tip. I'll look into it. It just came to mind today that we have two forklifts at work that use LP with a carb. I'm going to see if I can disconnect the fuel line from the forklift carb and attach it to my motor and start it. Then I'll be able to know that it starts and runs.

It has 85# of compression on a slow crank. I wonder if there is a compression release in play, like on the Kohler engines, until it reaches a full rpm, for easy start purposes, because the engine has never been used and 85# seems low.

I took this engine in trade for used coal stoker parts and don't have any particular use for it.

Here is a real good info on the subject, read up !

Here is a link that might be useful: Propane & Natural gas

Here is another excellent reference site on gasoline, propane and natural gas powered small engines.

Here is a link that might be useful: U.S Carburetion conversion

Thanks for the tips. I have to put this on the back burner for a bit, so I can get other things done, but I will check into these sites.

I had an old (1950) Onan twin cylinder 3000 watt generator that was converted to use propane (LP) gas. I was able to run it at full load with only a simple barbeque tank and regulator. The conversion to multi fuels does not change the engine in any way (timing, compression ratios Etc.) Spark energy must be very high because LP ignites at a higher temprature than Gasoline. Replace your plugs if in doubt as well as the plug wires if they are not in tip top shape. Try a .025 gap on the plugs. It is also easy to over fuel (too rich) when starting these converted engines. See if you can have a helper crank the engine while you turn on the fuel flow. If you are trying to manual pull start this may be difficult. You might have success with a quick squirt of gasoline or starting fluid down the carb to get it to pop off before turning the LP fuel on. There is no reason you can't test the running of this engine on just gasoline carefully squirted down the carb. BE CAREFUL and do all of this outside. LP is heavier than air and can settle into low spots. Ignition of such trapped gas can spoil your day! You might try a bit of motor oil in each cylinder to increase compression. Your engine is probably dry from all that sitting.

Thanks for the tips Mark. I was just thinking about the engine the other day. I'll give them a try.

I just got a Miller welder with a ONAN P218G engine that runs on LP. The welder/generator engine came with the LP regulator but nothing to connect the regulator to the propane tank. I used the hose from a LP heater that has the LP regulator built in and I was able to connect the engine to a gas grill LP tank and run the engine. The engine idled a bit low and it would not rev. After a few more tries I decided to get a gas grill LP hose without the pressure regulator. It works great. The only modification I made to the LP hose I got was to enlarge the hole in the LP tank fitting on the hose. I enlarged it from .020" to about .060" and I could have gone larger as the engine has it's own pressure regulator and the engine needs more LP volume flow to run than a gas grill. If you're going to use a horizontal mount LP tank, as the ones on forklifts, you will need to get an evaporator. There's one on e-bay right now. The evaporator will take the liquid propane being drawn from the LP tank through the internal pick-up tube and allow it to convert to gas before it reaches the LP carburetor. The evaporator prevents ice buildup on the carburetor as the liquid to gas conversion takes place in the passive evaporator long before reaching the carburetor. I hope this helps.

This is a pretty old posting. Since then I was able to get the gasoline carb for this engine, but still haven't found the time to try it out.

I have a 16 HP twin cylinder horizontally opposed Onan engine that has a loose intake valve seat on one cylinder. I only need the short block, which I haven't been able to find yet but I believe yours would serve my purposes very well if indeed the short block is the same as mine. My engine came on a TORO/Wheel Horse tractor, new in 1987 and has a model number P216G-1/10976C and is SN L903752527. If they happened to be a match the propane aspect wouldn't matter I suspect.