Hi the separate black item is a heat sink for a 2N.... series power transistor. These were typically rated 30 -50 watts. If anyone wants any I have lots of as new NPN power transistors similar to this one.
However the heat sink is there because there is a 0.7 volts drop between the emitter to base junction of the transistor.
May not seem much but with lets say 5 amps shared current between the 2 off Monoperm super electric motors, this gives a 3.5 watts 'wasted' heat which has to be dissipated by the black heat sink. When I say wasted this is battery power that will not get to the motor.
Modern electronics have very little losses through heat and some esc's have no heat sinks on at all now. A few powerful ones have water cooling to get rid of heat.
I read another thread on this posting about sorting out the electronics but this is 4 pin operation and if memory serves this was a centre tapped voltage system and just not worth converting. Best advice is to pass on the item as a bit of history and get new controllers.
I see that the motors have Kardan couplings to the prop shaft, these are interesting as they expand to fit the gap, I do not think they are on sale anymore so take care with them. Boat looks in excellent order though.
Nice to see the old kit! I do have some old stuff but I go out to sail my boats not exercise the electronics, we are in a fit and forget era now, so we can get on and enjoy the boaty bits!