I feel your pain.
Recently I completed a 6 month tractor restoration. This was a complete strip down to bare castings and replacing every worn moving part, priming, paining and reassembly. Wired it up with a new harness - and it started but the timing was off - after giving the starter some quick burst to find the TDC markings on the flywheel -it suddenly grinded and screeched to a halt and the starter jammed up into the flywheel and it was the worst sound - breaking a number of teeth on the flywheel.
Needless to say - I now have to disassemble it, split the tractor, replace the ring gear, and reassemble. I put it off til this spring - Sometimes you hafta just walk away and exercise the common art of denial.
Hang in there. Hopefully you will track down the cause...
