Trolling Motor Speed Controls

[RidgeRunner]

Motor Guide 24V bow mount trolling motor on the flats boat. Circa 1996. It was purchased as a wireless remote control unit and converted to a hand model after multiple failures. It now has a bad circuit board in the head which is no longer available. When I isolate the wires to the motor itself I get high speed with 24V and approx half that with 12V.

Electric motor is rated at 33A so I would need a 500 to 750 watt speed controller. The factory used a printed board they labeled "MOSFET" to vary the voltage to the motor. I do understand what a Mosfet is but not at all sure of the architecture of the board nor how to repair it.
I am not an electrical engineer, but for well short of another $1000 bucks (replacement cost) I think I will find something that will work;-) Motor doesn't have 20 hours of use because it is always broke when I go to use it...

Questions--
1. Can a pulse/wave modulator be used to control speed?
If so what are the disadvantages?

I assume MOSFET is very efficient and suspect (from all the advertising)full analog controls to be harder on the batteries. BUT I may be able to adapt a DC motor contol out of a electric scooter. They have some real inexpensive variable speed controls available. Some come complete with twist grip (POT or Hall Effect) that would be an easy adapt to the hand control. Some are rated for water too.

2. Full digital conversion? Is it possible, what would it set me back if so?

Again, I am looking to stay cheap, this motor rarely gets used. As it is now I have a high and low. I can live with that but would prefer variable speed. Thanks in advance for any help/opinions. Type slow, I'm blonde....