There could be a restriction in the intake air box or the exhaust (or both). The vehicle may need to be re-jetted (carb) after the restriction is removed. Years ago I picked up 7 mph on a moped by...
You can, but don't expect to get much more speed out of such a tiny motor. They were just designed for riding around cities, not for road worthy speeds. How ever, if you really want more speed, you...