You should always tell what you have when you ask a question like that.
But in general a tuned exhaust pipe will give you 4 or 5 more mph.
Bi-Turbo and Leo-Vinci make exhaust pipes...bout $80.
Many scooters and mopeds are limited some by the type of transmission they have.. and will only go faster by revving higher.
Revving higher causes these small motors to blow up or wear out and break sooner.
No matter what you do.... before you modify it run the bike hard and remove the spark plug and look for the ceramic color...then when you start modifying it keep checkin the plug color.. if it starts getting whiter and whiter you are in danger of a seizure and need a bigger main jet for the carburetor.