If no one else is able to come up with a suggested motor size based on
actual experience, I'll suggest that you could do a little experiment
to estimate the power required. One hp is 550 pounds lifted one foot in
one second. Ref. https://en.wikipedia.org/wiki/Horsepower 

If you connected a pulley, wound a rope around it, and hung a weight
at the end, you could find the weight, distance, and time needed, and
compare that with a little algebra. Example: A 10-pound weight unwinding
11 feet of rope in 2 seconds would be 55 foot-pounds-force per second
(55 ft-lbf/s), or approximately 1/10 horsepower.

I'd use that as a starting point, as an average. Starting forces will
be higher due to 'stiction', and the force probably varies quite a bit
over a cycle -- I'd double or triple the value your experiment gives.

If the organ has any value at all (and even if it doesn't -- but that's
just me), I'd also suggest that the modification be reversible, so the
unit could be put back into original condition.

Kenneth Christ