Dale has found a supplier of motors, and of especial interest are small, slow speed motors. The supplier is www.the gizmoszone.com:  this page shows an 8mm diameter 25rpm motor.

There was a reference recently on the model-boats.com website to another supplier: Precision Microdrives

One of the tricky, early tasks in building a model of a twin-screw vessel is fitting the propeller shafts and tubes. Whether you are building a kit or from scratch, it’s equally tricky because you have to locate and make two holes in a surface which is probably sloping in two directions. The holes need to be in precise locations, and of a precise shape. They’re not even circular, but are oval. To make it even more complicated, often your only reference points (the ends of the propeller shafts) are located in mid-air somewhere, and you cannot see from one to the other in a straight line.

Installing twin propeller shaft tubes.

Building tips - Hardware

Confederation Marine Modellers

Hints and tips - Small, slow speed motors.