Just got a couple of small gears and pulley from here:
This is a pretty nice supplier for the components, other surplus sources should be checked as well, but for smaller motors (3mm shaft) like those I had ordered like half a year ago are quite hard to find. The above supplier is nice since they had no minimum order amount and the shipping cost is reasonable, for my order, it costs only $104 HKD shipped, and it's already the cheapest source I could locate.
I will use the pulley or gear, together with a belt to drive focusing knob via a DC geared motor. The JMI hand controller will be used, and a mono audio cable jack will be the interface, and so it's basically plug and play. By cutting a mono audio cable into two, you can have interface plug and cable for two motors.
I plan to glue the motor onto the 2" visual back, or maybe using those hose clamping to fix it in place. Let's see how it works out. I guess it would take 2-3 weeks to arrive.
Additional Remarks on 15th of June: the items were sent on 11th of June, expect two to three weeks for the delivery and then I will begin my work!
Additional Remarks on 17th of June: the items arrived, well packed, fast and good. I shall try to fit them on my mini dc geared motor tonight. It seems a good fit judging by my eyes, and the pulley seems nice and what I have in my mind. The motorizing project should be quite close now.