Addendum to my previous reply - I removed blower shroud, and saw that there are two separate coils for this motor, rather than the duplex coil that was on my other engine. Any way to check for a bad coil, other than visually noticing a severely delaminated armature or burst winding casing?
I also looked up the air gap (.20 - .30 mm), which seemed OK on the motor.
I checked compression - 115 lbs on dead cylinder, 215 lbs on hot cylinder. I squirted some Marvel Mystery Top Oil into the dead cylinder in case of a stuck ring. This time, when I only ran the motor enough to back up the tractor, I found the dead plug very wet, and the hot plug full of black carbon.