Re: If You are rich
With a large amount of play money I'd build from scratch a two stroke Diesel, 5 cylinder rotary radial of about 140 cc's displacement, built into a rear wheel. It would weigh 20 pounds, get great fuel mileage, and make the most awesome open pipe racket when pulling hard on grades. It would have a epicycloid 5 speed transmission with wet clutch and gears.
It's a motorized bike, dream large!