Judging by their site, its probably a Polini 50cc 2 stroke dirtbike motor (judging by the carb, thats what it is). Automatic, with centrifugal clutch running directly off the crank with gear box, to the counter shaft. Or, its a honda 4 stroke XR50 or 70 motor, big bored and stroked. Stock is a 3 speed w/out a clutch. With some mod's can be made to a 4 speed with hydro clutch. I've seen them put out close to 55hp in the 70cc motor. You can also run a 2 stroke carb on these as well. Usually a KX65 carb is whats used.