I've always found this intriguing and want to build one some day. I'm curious though if it could be made without the water pump. I seem to remember that back in the late 70's or early 80's when the water cooled motocross bikes came out that the Kawasaki KX bikes did not use a pump. I don't know what it's called, convection, thermal dynamics, whatever, but the concept was that hot water would rise to the radiator and the cold water would sink back to the motor.
I don't know how well this system worked for a high revving motocross bike, but for what we're talking it might be perfect.
