[lincpimp]

A 9l is a good 3s motor, you can probably stick with the 13t and be fine. Maybe go down one tooth if you want. Plugging the values into BrianG's calc (linked in the post above) will get you an approximate speed and then you can change the tire size and gearing to match. I have run 9ls on very heavy pede based 2wd projects and always had great success, even geared for 50+mph!