Yamaha r125 lost speed. HELP ME PLEASE!?

I have a 2011 plate yamaha r125 which I had an over 75mph top speed, but recently I had a service for new oil and filter. Whilst the service took place I had my foot gear lever taken down 1 space on the spline and had a scorpion exhaust fitted. Now top speed is only 55mph full throttle and slows on small hills. Btw I have left the baffle in the exhaust. Please could anyone help me ?