Spark Magnetos

Sounds to me like one of the valves is stuck open (probably the exhaust).

First thing you need to do is make sure the #1 plug is getting fire. You could have a worn point cam on the #1 side which isn't allowing that plug to fire (odd but not unheard of). Don't do this in full sun as the sparks are hard to see, best to wait for dim light rather than dark so you can still see what you are doing. Pull all the wires off the plugs and ground #2,3 and 4 so the engine won't catch unexpectedly. Put a spare plug on the #1 wire and ground the plug threads to the engine so you can see the spark gap (if you hold the plug to the engine you may want to wear rubber gloves ;-). Have someone crank the engine over a couple of times and look for the spark (you can do this on any of the others to see what the spark should look like.) It doesn't sound to me like this is where the trouble is but its a good place to start. If you aren't getting fire you may want to switch to an electronic ignition which saves you from having to replace the worn distributor.

Next thing is to check the compression on #1. If its present but low compared to the other cylinders you may be getting excessive blow-by and diluting your engine oil with gas. If it doesn't show any significant compression at all, pop the valve cover and look for a valve stuck open. Sometimes all the tractor needs is to be worked hard to "fix" a sticking valve. Other times you'll need to pop the head off and have a valve job done. In either case you should pop the valve cover to be sure something isn't bent or broke.

Good luck!