Hello,
Most of us convert engines for radio controlled airplanes. The engines will work in a plane, boat or car application. You just need to know what sort of drive system is needed. Such as a prop, gearbox or shaft and screw for a boat. Boats and cars work better with a clutch. You want to retain the fly wheel cooling fins and plastic shroud for boats and cars. Of course boats can use a water jacket instead of air cooling. The pull starter would also be helpful with starting surface vehciles.

Dave