weird problem moving away from stand still

well yesterday my bike (honda scv100) was fine untill it started going very slow on the slightest of hills so turned round n wen back n it went down this hill pritty fast but everytime i slowed down it was like its stuck in a high gear n moves away slowly from lights and junctions,, got home and took the bolts off that hold the case that has the kick start on and the thing was stuck to the engine case its self shows u how long this thing hasnt had a belt change for lol so just bought a belt and rollers for it but do you think i should also get a clutch for it ? and before any 1 says about it being slow has it got enough petrol yes its got more than half a tank looking in the tank