When I was a teenager we used Victa 125 engines for our home made Go-Karts and Mini-bikes. On the earlier engines you could make the ports larger and increase the size of the cutout on the piston skirt to accommodate the larger ports. Shaving the head was a way of increasing compression. You might also want to tinker with the ignition timing and perhaps try different exhaust pipes/mufflers. These engines are pretty tough and you may shorten the life of the engine with modifications.
PeCo
