I thinky means to make it a drop-in kinda deal. So you could have a
chain drive hobbit motor.
Would you still need a gear reduction? Ohh wait no you'd do that with the sprocket sizes...
Shit you could just your in a scooter rear variator and bearings...you could probably use pillow blocks...problem would be the spacing of the drive sprocket, you would need a jackshaft to be able to line it up with the wheel....or could it fit behind the variator....hmmm