That engine has a step crankshaft 5/8" with a 5/32" woodruff key stepping down to a 9/16. If you decide to inboard mount the clutch for the mini-bike I can build you a 12 tooth clutch that will be fully supported by the crank. Only thing I would need to know if the threads are left hand or right hand. Most are left hand but not all of them but the same threads 7/16-20.