I haven't done this myself, but I *believe* it can be done on the rear wheel, though there may be some work involving changing axles, bearings and shoes on the back. As for the front, you're looking at an 18" wheel instead of the R90's stock 19" wheel, and whatever modes to be needed to make the front rotor align with the swinging caliper ATE brakes I'm not sure - - THAT wheel change would make a big difference in steering (and I'm not sure it would be good?)