Expansion exhaust to get more pep outa that 50. Anyone will do just have to match it with jetting. Ive never had the stock cylinder however. Mine came with Airsal t6 which for 50cc is crazy fast with pipe. If you want to be stealthy get Malossi 70cc kit and mod stock exhaust.
Mine is modded stock exhaust, stock carb with aftermarket filter and 105jet, NCY race clutch, stock var, Malossi 70cc sport cylinder. I get 45mph and its tolerable noise wise. ALOT of low end power this way and if you can believe it pretty darn good mileage.