I've had this gearbox. It worked ok with TLE43 Mitsubishi 2.2hp engine. You could also use the Tanaka engines too. If you want 50mph potential, try the 4.5hp CY460 engine. Prepare yourself in changing clutch springs. If you want a dependable powerful engine, go for the 2.8hp Tanaka 47R engine.
However, if you can't fit a quiet expension pipe, the last two engines are VERY loud.