Moped Army

I have a 3 year old Yamaha Aerox. It was going fine until about 6 months ago when the exhaust blew a hole in itself and I got a Tecnigas powerpipe fitted as a replacement. Over the last six months the bike has been getting progressively slower especially on uphill. The revs are still at the same level but the power is simply not there. I changed the spark plug and cleaned the air filter but that didnt help.- any ideas ?
Thanks