Well did you fit a bigger main jet in the carburettor...what size is in there right now?...with that new exhaust I would fit a bigger one
also there is a wire connected to the CDI that will require earthing to the chassis or battery negative, this wire goes to one of the plugs at the speedo, it is black/green ...make a connection from this wire to chassis. This should have great effect.
Dont forget though...fit a bigger jet in the carb or you will be back with photos of a melted piston!