The 7 HP is to large, won't clear the chute.

Any engine you choose you need to verify crank extention or length out of the engine. Snapper uses the longer crank, something like 3 5/32 long.

The woodruff keys. One is for keying or locking the blade adaptor from spinning on the crank. If your Snapper is a self propelled model it needs a second key higher up to lock the drive system pulley to the crank.

Does your engine blow oil smoke? If not, there may be other remedies for lack of power. A 4 HP should be able to mow reasonable-height grass while bagging. I have a 1989 3.5 HP that still does. The governor may not be working properly. The mixture may be too lean - your mower is old enough to have a mixture screw.

I'll look for the mixture screw. I did notice I had to add quite a bit of oil this last time. I never need to add oil to my other mower. I had only been using the Snapper to vacuum in brief, maybe 15 minute, sessions. Yesterday was my first full mow of the season.

I recall it smoked when I first got it but I haven't noticed smoke lately. It does take several pulls (at least 5) to get it started. The GCV starts on the first pull even after sitting for a couple months.

The Snapper is a self-propel.

I looked in my mowers manual (found it online) and looked up the mower model number and the engine serial number. It is a 3.5HP, not a 4 HP as I originally reported.

Starting to sound like it's just about worn out. If the rest of the mower is good, have the engine rebuilt. You're not going to get the Honda engine on there.

Nevada_Walrus is right on with the crankshaft length and the key descriptions. These are just the little half-moon shaped metal pieces that fit in divots in the crankshaft and stick out a bit. The drive pulley and blade hub each have channels milled into them that fit over the key where it stick out of the crankshaft, so that they don't slip on the crankshaft. In addition to crank length and key location, you will need to know whether your replacement engine has a 7/8" crank diameter or 25mm (I think the Japanese engines all use the latter). It matters because you will need a blade hub and pulley of the correct diameter to fit it. Snapper used many engines and so they have hubs and pulleys to fit either - check your local parts dealer. You can probably reuse the hub you have if you get an engine with 7/8" crank. But don't count on getting the pulley off - it's probably rusted solid onto the old engine.

Lastly, your current throttle and dead-man cables probably won't fit any new engine. Again, Snapper made Hi-Facs with Brigs Quantums and Inteks, most Honda engines, and both Kawasakis, so you have a lot of new engines to choose from and still get the proper cables. Plus, I guess many new engines are not variable throttle (full throttle only) and for those you wouldn't even need a throttle cable. None of this is rocket science. You can do it if you have all the correct parts.

Rebuild? See above regarding my limited mechanical ability (smile).

"Man's got to know his limitations."
Dirty Harry in Magnum Force, 1973

It might be fun to try though next winter after mowing season ends, or it might be better for me to save my money and see about a complete new mower.

Whether to replace engine or entire mower depends on the drive system condition. Inspect horizontal disc on deck, vertical disc that slides back and forth on hex shaft and diff for slop in the bearings. Belts and tires are expected to be worn after 20 years but those three major components can get pricey if they are worn as well.

One thing to consider is that your 1986 mower is 20 years old. If you are able to find a new motor that fits the remainder of the components will be 22 years old and nearing the end of their useful life.

You said it was self-propelled, does that mean a push power or does the mower propel itself. If it has a drive system that complicates the addition of a new motor.

If it were me I would use early spring as an opportunity to shop for a new mower.

Unless you want to undetake an engine rebuild on your own