Pull the front cover and count the teeth, we can assume (always dangerous) your rear is a stock 42T. If you want more get up and go off the line, install a 16T, a less buzzy (lower RPM) top end, a 18-19T. When I ran stock diameter rear tires, I preferred a 17T, the best compromise to me. It takes a 36 mm socket and a long breaker bar to do the changeout and a helper to stand on the rear brake really helps. I have a 3/4 and 1" dr socket collection, so I have it easier than some. It's a 10-15 minute job with all the tools in hand. Worry about tuning issues, then sprockets if I was you.
