Get a 7/8" bore sprocket and put it on the output shaft? If the shaft is about 7/8" long there should be room for a #35 or #41 sprocket and room for the chain to clear the side of the engine block. Then use a jackshaft that has a regular centrifugal clutch on it. The sprocket on the centrifugal clutch would then drive the rear sprocket. That might work and save a heck of a lot of trouble, especially as a jackshaft is really the way to go on any minibike.
sold my go kart,, looked on craigslist and found a doddle bug for 150 and the guy was throwing in a power washer with a 5 horsepower engine.. after i made the deal i found out it had a 7/8 shaft and was a little let down while i was thinking of an out. they guy called me cause he knew i worked on engines sometimes and he needed a carb rebuilt, while talking to him i told him i thought i needed a 1 inch clutch if i was going to make this work, he then said he had one and that we could make a deal if i worked on a couple carbs for him,, so i made the deal gave him 25 bucks and did 3 carbs. now after i got the clutch it was just a matter of making the 7/8 shaft fit a 1 inch clutch, the clutch had a built on keyway and had to be bolted in the the cent of the shaft,, there was no set screw on the clutch. well i found these and ordered one DAYTON Bushing, Reducer, Pk3 - Reducing Bushings - 2X653|2X653 - Grainger Industrial Supply now that makes my clutch a 7/8 diameter id hole, the 7/8 engine shaft is keywayed and only sticks out about an inch and a half, but that is perfect for the clutch cause that is all it takes of the shaft. so i plan to notch the shim to take a keyway from the shaft.. then i need to drill and tap the shaft so i can put in a bolt and washer to hold it on, i see no reason this wont work, then i will ride the mini till it blows, once it does i will fit the 1 inch clutch to a 8hp or something! :wink: .. so these gc160's with 7/8 shafts are not "junk" as most people think.. and if i didnt have the clutch i have i would simply just order this Collars & Couplings | Collars | Step Down Clamp-on Adaptor SDA-Series, 7/8", Black Oxide | B282692 - GlobalIndustrial.com and run a 5/8 clutch on it.