Seats
This is a relatively easy part. Although the seat is a fairly important part of the kart. what you need in a Kart seat is plenty of "side support", as well as a nice snug fit, so you will be able to steer the kart, instead of using the steering wheel as a rigid support.
The best seat around is of course a plastic seat out of a proper racing kart, even an old second-hand one. You might pick one of these up for half nothing in a karting place/circuit.
But if your like myself, you can always improvise. The next best thing to a racing seat is an ordinary plastic seat, with support installed either side of the seat. Take an ordinary plastic seat, bend the four legs out flat. Leave around 50mm sticking out from under the seat. Now either bolt or easier yet weld 20mm (3/4") lengths of pipe at either side of the seat, running from the top back of the chair to the legs bent out at the front bottom. The top of the pipe can be bolted to the plastic seat.
Then as in the photo, put some pipe insulation around the side supports, and secure with tape. Now all you have to do is to put the seat on the frame, sit in the seat. Adjust the seat, to the side, forward/ backwards to suit the pedals and steering wheel. When you are happy with the position of the seat, drill holes through the flattened out legs and floor pan, and bolt up with 6mm bolts and locknuts.
A Modified Ordinary Plastic Seat. A Racing Kart Seat.
The modifications needed to be made to the normal Plastic seat, for extra cornering support. All can be made from 20mm (3/4") piping, or anything at all. Bolt or weld together.