Looking for a new motor

GoldenMotor.com
I'm looking for a 66/70/80cc motor, whatever size you want to call it. I swear I saw a kit that came with 2 different sized rear sprockets, a 44t and a 50t.

Has anyone seen this kit? I remember thinking I would want to look at this one when I made my second bike.

I'm not sure what size sprocket I need exactly. I weigh 260 and want the best torque/top speed balance I can get. There are a few hills here, and a fellow motor bicyclist said his 66cc rode up them just fine with a 50t. My current 48cc with a 41t has a top speed of 20-25 depending on road and wind, but NO hill climbing ability.

My budget is $150-$200. Any recommendations? Links preferred.