Chassis
Every part of the go kart connects in some way to the chassis. It is possible to use wood for the chassis, but you will probably want to construct a welded frame from metal tubing. Tubing with a 25mm outside diameter will make a durable frame. Draw an outline of the chassis in chalk on the ground. This will help you measure and assemble the different pieces of tubing.
Engine
You can purchase an engine made specifically for go karts, but a lawn mower engine also will do. After you remove the engine from the lawnmower, you mount it to the chassis sideways. Take engine placement into consideration when you build the chassis. You need to have a suitable place on the chassis to mount the engine, or you can build an engine cradle.
Axles and Wheels
You need front and rear axles on which to mount the wheels. These can be complicated, especially in front where the wheels are also used for steering. Buy the axles from a go kart retailer. You can also buy specialized go kart wheels or use wheels borrowed from other motorized machines. But you may need to use a particular type and size of wheel, depending on the axles you choose.
Brakes
Go karts need to stop as well as go. You need a round piece of metal to act as a brake disc and two brake pads for each wheel that you install brakes on. You can fabricate these parts or purchase them pre-made for go kart use.
Steering
Metal tubing will serve as a steering column and most of the steering linkage parts. It may be a good idea to purchase these items as well, or at least use detailed plans on how to attach the steering linkage to the chassis and the steering wheel.
Seat
The driver must have a place to sit, and you need to design part of the chassis to accommodate a seat. You can modify several types of seats to mount to a go kart, make a seat from scratch or purchase a go kart seat.
Pedals
You can use metal tubing to make small frames that act as brake and throttle pedals or modify pedals from some other motorized machine. As with most other go kart parts, you can also purchase pedals from a retailer.