I just changed the top sprocket to a 20th/15 plate (from the stock 21T/13P) and left the bottom as a 49T/13P, and used a shim out of a 440 race sled (it's a smaller shim than the original one that was on top, keeps the chain vertical), Some of the 440 Doos ran a 15 plate over a 13 plate sprocket (stock). I think I used an 86 link 15 plate chain, same length as stock. I don't have the spacer part number handy, but your dealer should know, or others here.
PS, I think I read of a 51 bottom gear, that would have been nice, leaving the top gear a little bigger may increase the chain life. A hyvo setup was my first choice, but I could not find one a couple years ago,,, but the 15 plate chain has never blown (about 1500 miles),,,
