if you really want any more speed if you can get a larger carb installed correctly on it i would do it. there are three types of restrictors available on mopeds. the exhaust system is calibrated to restrict airflow, the carb is too small, or there are actual physical restrictors in place on the moped. i wouldn't recommend going with the 60 or 70 cc cylinder/piston because a few of us here in bethlehem have had troubles with them. cheap materials. but the carbs/biturbo exhausts we had from them were great. but we couldn't install them without the larger cylinder crap installed at least without modifications that we didn't want to make. i wouldn't worry about losing acceleration because i don't think it will be that much. if i had to make a choice higher top end i think is more valuable. but that's just a matter of personal opinion.
that was on my tomos. on my 79 moby i'm sticking to stock. the way it was meant to be is the way i'm keeping it. rock.