Just ordered one 12V 5RPM reversible DC geared motor with 6mm shaft. Since it's slower than the ones which I had on hand, it should be even more suitable, and the diameter of its shaft matches the one from the TG-SP II exactly, it's going to give far less trouble. This one will be used on the DEC for motorized manual guiding first.
It has M3.5 mounting holes and should be easier to find than the M1.5 of the older motors.
Once it's proven to work, I'll buy one more for the RA.