The 85/85 engine needs a special "twisted skirt piston" because the cylinder is twisted on the cases. Does the ends of the crank use the same spline for the gear that drives the clutch? Are the cranks the same width and outside diameter?
I assume you are wanting to swap cranks to save money. If any of the above are an issue, you will spend more money making the late model crank work than it would cost to buy or rebuild the 85/85 crank.