My moped isnt running with the power it used to?

i just bought a used roketa maui 50cc with 700 miles on it rode it b4 i bought it ran perfect i rode it at my house then 1 day later i rode it and then it lost lots of power accellerates real slow but i can get it up to top speed the same it just doesnt accelerate or have the power that it used to it doesnt go up hills like it use to either. the owner let it sit for like 3 yrs if that helps if any one knows what the problem please let me know thanks