Converting to LifePO4 batteries will get you to your top speed faster, but likely not change the top speed; that's controlled by your motor controller and the information fed to it by the speed sensor. You can try a "high-speed" magnet, and that may make a difference (reviews are mixed on the effectiveness, tho). I have the Navitas 5kw AC motor with 600a motor controller in mine, and it's fantastic. I get 31mph (and enough torque to lift the front wheels significantly off the ground if I stomp on the gas) with no problem. If you're moderately mechanically inclined, you should be able to install the Navitas system yourself without too much difficulty, just make sure you also get a heavy-duty solenoid and a set of 2-gauge cables. The solenoid is definitely worth paying the premium price for the name brand, and the cables are best if you can buy the wire and make them yourself, but something similar to what I linked to will probably be fine, as well.