Honestly, get a bettter hull. A hull designed around a 4s2p rigger could easily accomodate your current 6s 1521 power system. You will be able to exceed 65mph and should be over 70mph. There comes a time when you will want that extra 5mph, why not start now before dumping more money. Build a hull toss some hardware on it and re-use your current power sytem.

Ryan