ps, if you can give me your exhaust port open duration in degrees as well as your peak rpms, i can give you the dimensions of an expansion pipe as per specifications of jennings.
i tried it out the other day and made a basic template in Pro/E in which i can enter those two pieces of information and it will return a "properly" dimensioned pipe. i say "properly" because i havent actually built one, but the dimensions are closely based on what jennings describes.
this is a pic of what i came out with. it doesnt look like much, but the dimensions are legit (for my bike) and can be easily adapted for different numbers. i left the header just straight cause god knows how youd want it for a given bike.
i think this could be useful to at least get an idea of the sizes that would work for your bike, then you can go around and look at pocket bike pipes or something and compare them.