Nice thing about pvc is that it is pretty soft and easy to work with. I built my manifold in place and then coated the interior with KBS tank sealer.
I am running a 34 mm carb at present the stock was 40mm. At 80 mph I am 1/8th throttle. I am sure that the bike would go 80 with a 20mm carb. Probably with more economy.
A simple Y shaped manifold made with some PVC 1 1/4" sink drain would be about the right diameter for your 200 cc cylinders. Just butt glue the parts to make the shape that you want. Then take the assembly and spray glue the reinforcement material to the plastic pipe wet it out with epoxy and the wrap the thing with strips of polyester or rip stop nylon cloth. The cloth can be peeled off once cured. You don't have to use carbon fiber regular fiberglass will work.
All that said if you get the bike to run stock you will have a fairly well engineered bike that should save you some gas.