not even close to a 'simple swap'
R65 uses a short stroke crankshaft, all others(750-800-900-1000 airheads) used the same crank throw and are somewhat 'interchangeable'.
I once had a project in mind to use an R65 block and crank, and machining(shorten) a pair of R800 nika-seal jugs to come up with a short cylinder 725cc WERA Vintage 3 class racebike. Last couple seasons I've been spinning wrenches in the AMA paddock and all my 'personal' projects are sitting on a shelf in my shop.
If you have access to machine tools its possible, but on top of fitting the jugs you have to be able to work out niggly little problems like pushrod angles and such.
Triple M Engineering