William,
The only spring in the engine accessible from the cam cover is the ACR spring, or Automatic Compression Release spring, and is located on the side of the camshaft gear. If it wasn't the dipstick cover but the points cover, there could very well be a bad spring on the points themselves, and he could get it to run by holding the points down on the cam, instead of letting the spring do the job.
Points will set you back maybe $10 or so, and take less than an hour to replace and adjust with simple hand tools.
If it is the ACR spring, then it might be a little more complicated. If your uncle saw that the "hook" on the spring simply slipped off of the counterweight and he put it back on, you might already be good to go. If the spring is broken or badly bent out of shape, it will need to be replaced Someone with good hands and some experience might be able to replace the ACR spring without having to remove the oil pan, which would mean removing the engine from the tractor, but from what I could see on a junk camshaft I have, it is a pretty tricky operation working through the cam cover opening alone. Pulling the engine is a pain in the neck, but you're less likely to end up with your new spring at the bottom of the oil pan, or running afoul of one of the moving parts inside the engine. At least the spring can be replaced without removing the camshaft or crank, which requires almost complete disassembly of the engine. If you had to go that far, you might as well rebuild the whole engine while you are at it.