Need a small speed control motor with forward and reverse? I built an RC 1/35 scale LCM-3 from a Trumpeter "static" kit. The whole boat is about 17 inches (432mm) long and the "engine compartment" is roughly 4-1/2 X 4 X 1-1/2 inches. In that compartment is the receiver, rudders servo and linkage, 4 AA cell batteries, and two independant motors with speed controls/reversing.
The motor and control are old (very old) Futaba servos with the gears removed. It runs great! Even the propellers were part of the kit but mounted on the short stuffing box shafts.
Given the limited space in this LCM, I'm sure that this method could power a PT boat, allbeit not very fast.