Gilera Stalker Problems?

Hi,

My son has had his gillera stalker derestricted by removing the variator spacer and a couple of other things at the bike shop he bought it from and he said the performance was good, I fitted a TECNIGAS exhaust for him and changed the rollers to the ones supplied with the exhaust and the performance is poor, he has trouble pulling off from a standstill on any incline and top speed has dropped, anyone help please.

Cheers,

David.