[Christ]

Depending on how light you plan to make it, and how fast you plan on going with it, the rear swingarm/solid axle assembly from some 4 wheelers might suit you. (chain driven ones)

My friend's Yamaha Warrior YFZ600 will go about 67 MPH tops, with a solid axle and the stock swingarm, and about 300lbs of dude on it. And we've been known to put street tires on it and drive it to work during the summer (keeps miles off the cars and the huge truck he has.)