The stock sprocket should be a 37 as stock. To my taste the 40 rear is a bit too much. If you want to try that ratio, you can change the front to a 14 for about $20.
We have the 39 sprocket in stock for your bike in the AFAM brand (i think these are about the best you can buy. I like the steel AFAM for longevity. $49.10 plus shipping from us.