I know if you get a F2 or F3 rear caliper support, you could put a dual piston CB900 or 1000 or 1100 caliper on it. It will fit, I can't remember which section it's in (FAQ or tips and tricks) but member Jaguar did a thread on the upgrade it's all bolt up stuff. I've considered the upgrade for my F3.
EDIT it's in the tips and tricks section, just look under brakes. I think it's the 3rd or 4th thread down.