If you have to start with the choke on then you are likely not too rich. Requiring choke to start is not a bad thing.
Lots of things could be slowing you down. Bad wheel bearings, dragging brakes, variator may not be opening all the way. It is always the best policy to get a bike running really good before adding performance parts.
That being said. If you can find a pipe then that is the easiest way to get more speed from a PA50. Since pipes are hard to come by for the PA50, you may have to hack something. If I wanted a pipe I would tecno circuit and cut and weld a header to match:
Another pedder in here had great success doing that on a honda express.