I have been tuning on expresses pretty hard this year. My personal experience is you are going to be best off with a carb in the 16-19mm area, and probably either the treats jamarcol pipe, or fabricate something out of a pocket bike pipe. They are super cheap on e-bay and if you cut up your stock pipe, take the header and weld it to the expansion chamber of the pocket bike pipe, it'll get you another 5+ mph. Both Rick, myself, and a guy that posts as expressmentor have done some pretty good right ups on express tuning. search around the performance tuning forum for stuff like nc50 mod, express speed, that kind of thing.
But yeah, stock carb is not gonna work for a kit. I can point you in the direction I took if you want to know.