You can buy a rear sprocket kit from any one of several sellers on ebay or with links here on the left.

#41 chain is available at Tractor Supply Co. or similar stores, motorcycle/mower/toy shops.

You will have to brainstorm up a mount for the bicycle motor that will allow a clear shot to the back wheel/sprocket. You'll have to modify the throttle and make sure you have adequate brakes, you'll need them. Don't be afraid to modify/remove/adjust the cover over the clutch on the power unit.

Git buildin'!

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If it ain't broke, and you mess with it long enough, it will be.