I am assuming that you are using a Chinese HT in frame kit.
What kit are
Explore this site, you never know what you'll find.
Here's a little jump start on your quest: A Guide to Bicycle Motor Preparation And Care
As for a sprocket suggestion, that depends on the terrain around where you ride and what you expect out of the bike.
Will you be encountering any hills?
What top speed do you expect?
(Note: gearing for faster top speed = less hill climbing ability unless you run a shift kit)