Peugeot Trekker scooter loss of speed?

Hi, I have a Peugeot Trekker 50cc 2004 model which has started to lose speed. When I pull away from a junction I have to paddle it to get going, the acceleration has become very sluggish. The bike has had all the de restriction mods done (rollers exhaust variator etc) and has been running great until recently. I wonder if its transmission rather than an engine problem as the engine runs and revs fine. I've just replaced the drive belt as it shredded the last one, which wasnt a genuine Peugeot item but I was still surprised disintegrated in such dramatic fasion. I would be greatful of any suggestions. Thanks.