I second the idea of using a DC motor with electronic control for
replacing any other drive motor.  The new DC controllers are amazing
and will maintain a constant speed with varying load.  Since you can
operate a standard DC gear motor in a wide speed range, you just have
to find something that will be vaguely close to what you need.

Changing sprockets and choosing a motor that is more than adequate
in terms of torque (which means just about any gear motor you can
find as the demands of a spoolbox are minimal) and is faster than
need, you can control it with an electronic controller and not worry
too much about buying an expensive motor that is exactly what you need.

The range of suitable motors is greatly expanded when you can
control them electronically.  If you are a fanatic about tempo you
can even add a tachometer (specifically made for linear voltage to
speed consistency) or just a good quality little DC motor connected
to a resistor network and a cheap digital panel meter and get a quite
accurate digital readout of tempo.

Best regards,
Spencer Chase

 [ But what about the noise of the gears?  An airmotor is much quieter.
 [ -- Robbie