I don't know anything about off the shelf things like gear boxes and such. I had made a friction drive junker back when I was a kid and after reading about some (2) of the friction drives here, I decided I could roll my own for cheaper and I'm all about cheap as long as it doesn't result in poor quality. I use a hub for a weld on sprocket without the sprocket as the drive wheel against the rear tire. It is mounted on a jackshaft with a pulley driven by the slip centrifugal clutch on the engine. I have an idler pulley through amazon to keep the belt tight without having to mess with slotting motor mount holes to slide the motor around to get the belt tight. One thing about the weld on sprocket hub. You'll need to grind some grooves in the face that drives the rear tire to avoid excess slipping.