thats a tough one
I think I'd try putting the shaft in a vice and closing the vice on one end of the key to prise the other end up like a see-saw.
You'd need to put a piece of hardwood or plastic in the vice on the shaft side to protect the shaft .
if the crank is still in the crankcase then you'll need to remove the vice from the bench and take it to the scoot.
Failing that you could try wrapping the end of the crank in freezer gel-packs for a couple of hours then heating it up with a propane torch see if that lets you drift it out.