When I built a 6.5HP 196cc tricycle for a friend, it would pop wheelies REAL easy, (too easy sometimes) and go 35mph. If you can figure a way to mount a Honda Clone 6.5HP engine, that would work. Instead of a transmission, you just use a centrifugal clutch and pick your gearing wisely. If you have enough torque, you don't need a transmission with changing gears.
Since you want a trick bike, you can gear it really low to go only 30-35mph max, and take the RPM governor off to get nearly 8HP.
Electric-Start 6.5HP 196cc engine:
Gas Engine 6.5 HP OHV Electric Start | Powerland
I'm crazy, and like it that way, I'm ZnsaneRyder
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