drain the gas, and change the oil as you already mentioned, remove and clean the carbs and re-install, replace the fuel line(s) and fuel filter if there is one, check the air filter and intake for rodent nests and replace as needed (you can temporarily remove for the purpose of trying to start it initially, but don't mow your lawn without it), check exhaust for rodent nests and clean out as needed, charge or replace the battery as needed, check the spark plugs, and gap or replaced as needed, add fresh non-oxy gas and see if it starts.

if it starts, then inspect the belts for warn out or cracks and replace as needed. Run through the gears to make sure they all work, and check the blades and sharpen or replace as needed. I'm sure there's more, but this is a good start.

I would remove the spark plugs, and squirt just a little oil in each cylinder, and turn the engine over, a couple of times. Reinstall the plugs. Instead of draining the gas - I'd pour a small amount of SeaFoam (any auto parts store) in the gas and also add to the engine oil. As Jim said - I would use some starting fluid - give a squirt with the air filter off.

I haven't even checked to see how much gas is in the tank (went over and got the battery to charge today and forgot to check it). Would it be a bad idea to try to turn the engine over with untreated gas still in the lines if I'm not draining the gas?

I would not drain the gas - it's messy, and hard to dispose of. Nor would I change the oil. Do the gas additive - SeaFoam.

Ok so we charged the battery last night and put it in today. Changed out the fuel filter, air filter, and spark plug. Battery was reading ~12v but the solenoid clicks once and then nothing happens. Headlights still work, but engine doesn't even attempt to turn over. We had a jump pack there and tried jumping it that way but the same thing happened. Out of curiosity I took a new battery out of his John Deere riding mower and put in the one we're trying to get going and it did the same thing (the new battery works fine - it's in the mower we use over there when we cut my grandmother's grass). Old battery did not work in the JD unless I jumped it.

Odd thing is that I put the new battery back in the JD (where it had come from) and all of a sudden it won't start the JD anymore unless it gets jumped (and then you can't disconnect the battery pack without the mower shutting off).

Any thoughts? Now we have two lawn mowers that won't start.

Also, I could turn the engine by hand, so it's not seized up. Cleaned a mouse nest out of the air intake and found one stripped wire (we're assuming the mice) but it was not broken, and we taped it up. Looked ALL over and saw no more damaged wires.

Got it running - banged on the starter with a hammer

So, it started on old gas or did you empty it?

I had a generator that sat since 2013 with gas in it that I fired up a couple weeks ago. I never leave gas in things but stuff happens sometimes. I drained out what little was left in the tank, AND removed the carburetor drain plug (very important). While it was empty I cranked it with a spritz of starter spray and the engine caught, so I knew it would probably run. Fresh gas and it started up and ran fine.