[cgtech]

You likely have a "resistor coil" system, not the more modern "solid state motor controller" (you need to verify this, post pics under seat?). This will limit your upgrade options without spending some $$$. You already have a "series" system, cant convert that. You could just change out your 6v batts for 8v batts, that would get you 33% more speed and torque no matter what motor you use (stock motor would be fine). Increasing voltage is the easiest way to get more speed with a "series" system. The drawback to this idea would be the resistor coils would be forced to deal with 33% more heat, which will eventually kill them (so start saving for a controller).