My neighbor's MB10's crank broke at the keyway too.
Turns out clutch is 16mm and PTO on the engine is 5/8", so the clutch is fairly loose on the crank. I think that's contributing to the problem. We tested by fitting the Motovox clutch on 97cc PTO which is 16mm and it fits perfect. If it was 5/8" ID, it wouldn't go onto the 97cc PTO.
Has anyone contacted Motovox about this issue? If so, what did they do about it? :shrug:
If he gets engine repaired, I suggested he install 5/8" clutch.. I think Motovox is using the wrong sized clutch with 79cc engine.