Re: biggest motor/ hp with shift kit?
i would do a single speed and a large rear sprocket to minimize the load on the bearings, and single speed would allow you to use thicker chain. forget putting 4hp through bicycle chain and sprockets if multiple speeds is what you want from it. maybe cvt to jackshaft?
some people say that a motorbike defeats the purpose of a bike. i tell them to get a flintstone's car.