how can i fix my peugot speedfight 50cc lc?

i have a speedfight 50, its done a fair few miles but about 1500km ago a new airsal 70kit was fitted. it also has doppler race exhaust, uprated variator and clutch. ive been having problems for a while and cannot solve it, the bike has a good top end once going but no pulll away and looses power very easily, ive tried new belts, all light and heavy rollers and none of it seems to make a difference, ive been told to check the compression but it reaches 8bar, is there anything else you can help me with?? many thanks.