What you have is an 18.75:1 gearbox driving a 15:16 ratio sprocket/freewheel. This gives you a total reduction of 20:1. The 7 speed freewheel (not a cassette) on the other side of the SI hub will determine the amount of pedal assist ratio you can give to the engine.
Most of the 4 strokes one could attach to the gearbox obtain max horsepower around 7000 rpm. Therefore with this setup max speed on level ground would be around 27 mph.