The rear wheels on all the airheads are 18", so getting an 18" spoke rear wheel isn't that hard to fit on your R65 (Assuming you have a dual shock R65, if it is a monolever, you'll have to look at possibly a rear wheel from a fairly rare bike, the R80ST.
Likewise, you could possibly use the front wheel of an R80ST - I think those were 18" - or if not, use the hub from one and relace it to an 18" rim. The hubs of the earlier, spoke rimmed airheads are different and would require more tweaking of shims, axle diameters, rotor carriers, etc. It can be done, like anything, if you've got enough cash.