If your tapers clean and goodly you shouldn't need the key. Basically never ran them on my mobys...or was it vespas? Maybe both!lol
Regardless if the flywheel taper and crank taper are in good shape you'll be ok methinks. I'd be surprised if the key is in the same place for both bikes.
Wait, diddnt a guy use electric start express case halves and crank to make an electric start hobbitt here a few wears ago? Or was the starter gear on the variator side.... huh