Aprilia SR 50 Street not running properly?

Hello, I have an Aprilia SR 50 Street with a Piaggio engine. When I bought the scooter, it had about 100 km/h on the clock. I've now replaced the much too loud exhaust system with a quieter Yasuni. The scooter runs fine, but it chokes when you give it full throttle, then revs up to full speed. On flat ground, it can easily do 60-70 km/h, and downhill, even more. Uphill, on the other hand, it can only manage 30 km/h, and I can't give it full throttle because then it stalls. It works at half throttle. What else do I need to do to make it pull better uphill and not overrev so much downhill?