Run a 9325 belt and experiment with variator weights from 30-39 grams given your stock exhaust. You can also add the shortest 14” rear tire you can find. Check your compression and if anywhere between 90-120, raise it 30 or so psi. Too rich a main jet will kill acceleration as will poor spark as will a poor seal between cylinder and exhaust. As will a carboned up exhaust. And of course check for rust and pitting on your pulleys in addition to oil or grease on pulleys or belt.
You also want the red clutch springs on a gy6 clutch. As far as contra, probably a yellow contra. I would think a heavier gauge contra would hurt acceleration. (edited)