Well it can be a million things....
Do you know the history of the scooter ?
If the scoot run fine before i suggest giving it a service with new filter and sparkplug and clean the carb.
It could also be time to adjust the valve clearance.
Maybe the main jet is something to look at too, but what Kermit said, it could be a lot of other things. Give us some background details of the scooter.