Without going into the engine there is little that you can do to improve the performance on that model. The performance however is pretty darned good as it is. I take it you are runing 24:1 fuel oil mix, if not please do so no matter what Bubba or Billy Bob tells you otherwise. That is a plain bearing engine and needs an abundance of lubrication. If I were you I would completely reseal the gearcase this winter and go through your ignition system completely using new plugs, points, condensors, plug wires and coils. The only thing you need to make sure of on the coils is that you get your spacing correct so they are neither too far away from the flywheel nor rubbing it. When youre ready for the project come on back with another request and we can help. You can increase overall performance by looking around and coming up with an old Michigan Wheel 2-blade peformance brass/bronze propeller. Those things were worth another 1-2mph over stock blades sometimes. Try the guys over in https://www.aomci.org in the webvertise section and see if anyone has one they will sell.