you could try a reverse cone it still improves the performance of your engine you said its a powertorque half crank if so its not a verry good idea to try and push excessive powere out of one better to find a full crank 125 lear how to do port work and increase the comp ratio with a reverse cone exhaust here is mine for example
made this engine for a gokart
with the right mods a victa will turn 10000+ rpms gthey were used as early kart engines for racing
