Take a chinagirl exhaust, cut the muffler off of it, bolt header to engine, slip on a flexpipe about 30-36" long, use a bicycle seatpost clamp of proper size to secure it to header. Add muffler to the end of it. Quite simple, can't explain it better than that.
Those "lot of mods" aren't going to do much for your engine. My brother has had a GXH50 for 10 years and didn't have any problems with it until he started over-revving it and then rebuilding and modifying it. The engine is pretty good out of the box, keep RPMs under 8K for best results.