Disabled internal electronics. Replaced electronics with a simple relay.
The relay, when combined with the internal push-button trigger, and film eject position sensor, replaces the entire electronics board.
The eject sensor is a switch that opens when the ejector is retracted fully.
The push button is normally open.
The relay applies power to the motor when the relay is energized. When the relay is de-energized, it shorts out the leads to the motor. This short acts as a brake, keeping the motor from coasting freely.
When you press the trigger, power is applied to the relay, which turns on the motor and applies power to the relay, through the eject sensor switch. When you release the trigger, the relay stays energized until the eject sensor switch opens the circuit, de-energizing the relay, and stopping the motor.
Why Not use a microcontroller
This project would have been a lot more complicated for having a microcontroller, and wouldn’t work any better for it.
I had an Agfa Commander laying around. I removed all the internal electronics and optics, and opened up the hole on the front of the camera, then mounted the lens.
Focus was achieved by inserting an empty film cartridge with a piece of frosted tape where the film normally resides. I retracted the original lens barrel with the lens set to infinity and looked at the image on the tape with a loupe, looking at a distant target. I then taped the original lens barrel into place.
Judicious use of epoxy and aluminum flashing tape to hold things in place, and to seal against light leaks.