Hi radiojoe. I suspect when the plan was drawn the only option was a small i/c engine. It was not so much the electric motors but the batteries.
The only small batteries then were ex-WD and they were low Ahrs lead acid there were some batteries about 2 amp hour for models but first they had to be filled with acid kept upright and then there was trying to find a charger.
It was all DIY. Bobs boards were a distant furure when proportional RC was available at a low price.
We are talking single switch controls on the Tx and 45 volts for the Rx. tuned reeds and no crystals.
I started off my first RC boats with dry batteries for driving the motor. I had a relay sequencing system to give fwd hallf, fwd full. stop. rev half, rev full, stop. I used U2 cells now classified as D type. You had to have a good memory but as the boats did not go that fast, time was not so much of a problem. I am talking 1958, so quite a while back now.
Regards
Roy