get some thick square tubing. steel, probably 1 1/2 x 1 1/2. a flat frame with several uncut square tubes. also when you weld the joints you should make sure that they are clean and strong
if you are going to make it two seater, make sure that you even out the weight distribution (where the engine is placed / bulk weight of go kart) so that it runs smoothly and all of the tires have traction.