Stone,
It is possible that the crank will work in the '79-'81 RM60 as well as the RM80. The bore will definitely be different and the stroke may be as well. The difference will be in the connecting rod. The bore will not matter, but the stroke will. Look up the Bore x Stroke specs for both models. Ypu may be able to use the RM60 crank with a new RM80 rod. I have several RM's and the cranks on some models will work in others, with the correct rod ('77 RM125B and '77 RM100B share the same bottom end).