Step 1: Frame
Essentially, it is made out of pine, which is light(ish). You want a main board stretching from front to back for support. The seat is up to you. It could be a plastic chair with the legs cut off. The engine mount should be sturdy
Step 2: Front
Next, install the wooden front axle. I used u-bolts to clamp the metal axle (the wheels are attached to it) to the wood.
Step 3: Engine
Step 4: Rear Wheels
Next, I went to Lowe's and got a 3/4" steel rod for my axle. I inserted bearings into wood and slid the axle through. Then, I put adjustable retaining rings on the inside of each bearing to keep the axle from slipping.
Step 5: Drive System
I bought a sprocket and a Max Torque clutch and a #35 chain. The sprocket is actually bolted to the hub.
Step 6: Throttle and Steering
I just installed a simple throttle handle directly connected to the throttle. The steering handle gives you better control than your feet do.