Here's a fantastic conversion:
"BMW R100RT 1981 (Diesel)
This one of a kind bike has a John-Deere/Yanmar Engine ( TNA-72, 3 cylinder about 900 cc displacement.) It has a billet flywheel and bell housing so the original clutch fits. The transmission is original. The forks are two inches longer than stock to give additional clearance to the front wheel/engine assembly. It runs on ordinary diesel and B100 to B20. It has a top speed of 63 mph. It gets 85 MPG when running a speed of 55 mph. It loves biodiesel. I have repainted it, added a faring and the tail section around the seat, and have added duel tanks. I have outfitted the tubing with biodiesel compatible tubing and hooked up a valve to go from tank to tank. I have a spare transmission and spare engine block along with additional engine parts."