Adapting a go-kart with a motorcycle engine is a project that, while complicated, can still be accomplished with simple hand tools and a good plan. It's a good idea to avoid allowing young children to play on a motorcycle-powered go-kart based simply on the ease with which they could get hurt. For responsible adults, however, the thrill is intoxicating. Choose a small motorcycle engine for your initial build--250cc through 500cc single-cylinder or parallel twin-cylinder motorcycle engines are readily available from local motorcycle scrap yards, generally with the transmission included in the purchase price. More powerful, larger V-twin engines are far too powerful for this build, requiring additional structural support and roll cage construction. If possible, purchase an engine that you can hear running for yourself.
Remove the old go-kart engine using the socket set, retaining the accelerator linkage and mounting points.
Remove the go-kart transmission and drive belt or chain.
Determine the overall size and mounting position of the engine you will be installing into the go-kart. Ensure that the exhaust tube will be routed far enough from your head to avoid discomfort.
Use graph paper and a pencil to lay out how the engine will be placed in the go-kart. Ensure that the transmission final drive will line up with the sprocket on the rear axle of the go-kart.
Form the new mounting points using 3/4-inch square tube steel stock. Motorcycle engines tend to have three mounting points--two in the front and one in the back. Utilise all of these mounting points for safety. Test-fit the pieces according to the diagram. Because of the additional weight involved, ensure that the engine is mounted as low as possible in the frame.
Install the new mounting points by bolting them solidly to the frame or welding them into place.
Test-fit the motorcycle engine into place, ensuring again that the drive chain sprockets line up correctly. Bolt the engine into place.
Install the drive chain, new fuel line, accelerator linkage and clutch linkage. You may fabricate either a hand-actuated or foot clutch utilising leftover motorcycle parts. It may be easiest to use the hand clutch attached to the steering wheel for this purpose.
Install the starter switch and then wire an engine kill switch between the battery and spark plug for manual operation in case of an emergency.
Prime the engine with fuel, then ensure that the crankcase has fresh oil, and that the spark plugs are in good condition. Wear a helmet, then get in and start up your new go-kart.
Attempt to use as much of the old motorcycle as possible--including the starter, battery and even the throttle, clutch and brake linkage, if possible.
Always wear a helmet when operating a go-kart. Never allow children to ride on a modified go-kart. Go-karts are not for street use--drive them only off-road or at an approved track.