I motorized my 20-note organ a few years ago due to shoulder problems.
I use a 24-volt skateboard motor running [at 1/2 normal voltage] on
a single 12-volt battery. For the large pulley, I was able to get a
treadle sewing machine pulley which is just under 12" in diameter.

I made a 2" diameter pulley for the motor and a simple bracket that
lets the motor be removed when not needed. The bracket also allows a
small amount of adjustment on the tension for the 1/4" round urethane
belt. A set screw releases the large pulley allowing the mounting of
the original hand crank.

The 12-volt battery is hooked to the motor with a speed control. I have
two batteries and can exchange one while the other one is charging if
power is available. This organ can be played from rolls or MIDI files,
hand-cranked or with the motor.

Bill Klinger
Flippin, Arkansas
http://www.klingerorgan.com/ 
bill@klingerorgan.com.geentroep [delete ".geentroep" to reply]