I have just never seen that problem... I was shearing flywheel keys when I had to start the bike with a drill, but not since then. I would guess however that the flywheel is catching on something. clean up the fins with a file and make sure no pertruding points are left. did you include the spacer? if that was left out the flywheel won't turn at all.
IMO try this:
put everything back together very carefully with a new key, after you clean up the flywheel. clean up the starter clutch shoes and make sure they move freely. try starting again making sure to use the decompression valve until you are at good rpms.
If it shears again the flywheel is still catching something. a bent crank, the subframe being squeezed to tight, those gnarled fins, or a spacer missing under the flywheel is all I can think of that would cause this.
If the clutch starter shoes are sticking, you might have to pedal way to fast before they jam open, puting unnecessary torque on the key.
Good luck with that thing. here is a new flywheel. But we all seem to agree that yours is fine.