Running probem funtech 50, just bought it HELP PLEASE.

Hi all, new to here. Bought 2001 funtech 50cc this week for behind my motorhome. The problem is when on full throttle seems to hunt all the time, it is as if it has a restrictor than keeps cutting in, the revs surge constantly. The mileage is 6000 miles , it starts perfectly, ticks over smoothly, by the look of past documents has had a piston kit fitted.
Any help appreciated wether its carburretor on something more serious.
Also could anyone tell me which piaggio i would get spares for it, are they direct off the scooter, ie, gearbox carb etc.
All help really appreciated.
Ian