The new motor has arrived, it's a 12V DC geared motor running at 5 RPM, it's around $150 HKD each shipped. It has a 6mm shaft which matches the shaft of the TG-SP II. A surplus inline coupling ordered from the UK works perfectly.
I hand-held the motor and apply 4.8V DC to the motor, let it run, it runs very slowly and it drives the DEC slow motion control shaft directly. The DEC moves extremely slowly, barely noticeable with naked eye if you put a mark there to see the mark drifts, otherwise, you simply cannot even detect the movement.
So this experiment works, it confirms that:-
1. inline shaft coupling works
2. the motor is powerful enough to drive the DEC slow motion control shaft directly
3. and the speed is about right
Now, the next step would be to order one more such motor, and then custom made a small box which fixes both motors in place, and to put all the electronics (Solarbotics L298 kit based circuit) inside.