Hi Bob. I ran an RT40 with 2R then a 12R and used lots of parts from the 4R 1600S engine. I used the 12R as it had full flow oil flitering whereas the early 4R's had partial metered oil filtering. Wade, or Crowe 24/80 cams worked fine most heads had dual valve srpings, but the best thing you could do is find a 9R head Yamaha from the 1600GT and fit it. QED using blank dummy oil shaft and front timing cover. I ran a 48mm weber on a 12r with high dome 4R pistons and it won rallies.