I put a gas motor on mine... inspired by Pablo on this board.
My sprocket mounted on the other side of the rim, so I had two. It had only one speed. It cruised at 22 mph and could do 29 in screamin' mode. The factory brakes were next to useless, the vibration killed my hand. Not a problem with electric though.
They also have systems that roll a rubber wheel against the outside of your tire, or that have a replacement hub/ motor.