I have a very nice 1995 R100R Mystic and a 1975 R90/6. The R90 runs just fine, but the R100R engine is really sweet. Lots of power, runs very, very smooth, great response, etc. I prefer the ergonomics of the R90/6 versus the Mystic.
I had this crazy idea of taking the R100R motor out of the Mystic and putting it in the R90/6. I know it's a nutty idea because I would be trashing a beautiful example of a Mystic. But just out of curiousity, is there an "easy" way to do it?