Your going to have to build a custom frame so i hope you know how to weld. I'd go with a tubular chassis, preferably 2-3" in diameter, in the shape of a rectangle with an engine cradle on the back.
Can the engine be vertically mounted? If it sits at an angle it may be tough getting the frame at the angle it needs to be for the engine mounts. If you can verticalle mount it then you can run a chain to the axle right below it. I don't know if your bike had gears or not so I'm not sure how you'd hook up the clutch if it was a manual. Problem everything custom for that too. Same for the pedals and steering linkage.
Seats you can probably pull from a car but then your starting to add a lot of weight for a 200cc engine. I'd try to keep the entire go cart under 300lbs if you want it to be quick. Depending on how it's geared it should do about 50-55mph.
