Generaly everything is gove and take, you want more top speed then you have to give up some bottom end power. The 2 things to look at first would be your gearing or your exhaust setup. If your drive ratio is same as other people on same track and your engine is sam or close to same power band type of engine as they run then you should be good there, if your engine is more of torque based engine over local racers then you may need to just gear it up higher, if more of rpm engine then may gear it lower. For exhaust all of these can give you more top end power, shorter manifold/coupler length, shorter pipe, just putting a pipe on of same design but going from single to dual chamber pipe. With the gearing and pipe selections out there that should be enough to get you dialed in. Or you can try a hotter glow plug, or raise the deck height.