The type of motor you can run depends not on whether the fet upgrade is internal or external, but rather on the voltage and current limits of the fets that you use. In the datasheets, which is usually under technical aspects of the product, the manufacturer will tell you what the maximum amount of current a fet can either source( supply + to -, a npn fet) or sink (supply - to +, a pnp fet) the current limit tells you the max winds and turns you can handle, since these produce current draw in a motor. The voltage limit tell you how many batteries you can run max thru the fets look up the threads on winding to understand of turns, winds and gauge of the wire used in the motor affect current and voltage.

bob256