good project, I think Merv will be back on here and extiling the virtures and vices of the aircooled rd club of which I frequent. You would get a lot more advice from there.
not sure of the model you have, but the brakes are the same as the xs range from 250 to 1100. If you have a bracket on the left hand fork stantion you could use SR500 brakes which are left handed. The wheels normally come pre-drilled to take another disk.
even better, I think the aprillia rs125 fits and has stonking brakes on them (they fit the xs400 frames of which the rd uses the same headstock bearings)