A friction motor should have no gear reduction. A roller around 1" diameter should be mounted directly to the engine crank shaft to provide the proper gear ratio. 2 stroke China girl motors have a built-in 4:1 gear reduction. Using cave man math, you would need a 4 inch drive roller with the 2 stroke China girl to have the same top speed.
That is the main reason that you see goped style engines on friction drive bikes instead of China girl engines.