250s arent exactly sought-after motors. As such, theres not much of an aftermarket to them. But, as an inline 6 person myself, theres a few tricks. Bump up the timing a few degrees, port the head, along with the intake runners. Then uncork the exhaust, throw a decent carb on top, match with a coil and youll notice a profound difference. If your not happy with that, shave the head. I would be willing to bet your motor is running 8.6:1 tops. Chances are its leaning more towards 8:1.