The issue there is power. And suspension. Not enough, and not enough. About using that 1980 R100RS, ground clearance and suspension travel is not nearly as critical on a rig as on a solo bike for backcountry travel. I agree with Cleatus that unless yer gonna do some gnarly roads, all you really need is a stout suspension. That just means that if the shock is OK, you just need to upgrade the spring and then do the same to the forks.