The other night I did put the new spark plugs in, and then I tried cranking the thing over. First off, old spark plugs had a resistance of 50kohm, new ones have a resistance of 5 ohms. That's not a typo. Ok, I found ONE problem.
I tried pulling the thing over and I couldn't get it to even burble, even with starting fluid. Well maybe I put in too much starting fluid and flooded it, I don't know. So then that night I tried to go out to the tractor and see if I could visually observe a spark going between the spark plug wire in and the plug. I saw nothing, but given the fact that I'm trying to hold the spark plug wire in a fairly precise location and can only pull the thing a few revolutions at a time by hand with the other arm, that's not a very good setup. So I would say it doesn't confirm or deny anything.
At this point I've pretty much figured out that I need to get some kind of motorized thingamagig to spin over the pony motor while I'm diagnosing and trying to get it started. It could be that the spark is fine but the mixture is just wrong, and after sitting for 15+ years not running even though it has compression I expect it'll be a bit harder to start at least that first time. Plus I don't know what choke/throttle positions the thing is going to want to start, so it could just be that.
I'm debating how I want to try to make this work. The nut on the pulley for the pony motor is over 2" in diameter, really big. One friend of mine suggested an oil filter wrench might be a good way to do it since it doesn't need much torque it more than anything just needs to grip. That might not be a bad idea to accomplish the goal, at least for the moment. A new electric motor that would do the job from Harbor Freight is $75, which I don't really want to spend at this point when it's likely just for diagnostic purposes. Plus that's not how I would like to ultimately start the engine, given that a permanent mount would have the motor living outside and thus likely also have it leading a short life, as I seriously doubt anything from Harbor Freight is rated for outdoors.
I do have a starter motor pulled from a Ford 302 that has the power, but I'd have to figure out how to adapt the shaft.