Bruce Brockett said, "I'm planning to build a John Smith 20-note
busker pipe organ and would like add a DC motor to play it when
I don't feel like cranking."

I built a manually operated Smith Senior 20.  All Smith 20 organs
wind the roll onto a take-up spool.  Of course, as you play a roll
the take-up diameter increases and so the paper speed also increases.
Unless you have rolls which have been punched to compensate for this,
your music will go faster as the rolls passes.

For some instruments the rolls _are_ punched to take into account
the change in paper speed.  I don't know about all Smith rolls, but
I would guess that most are punched with the expectation that the
organ grinder will control playback tempo.

Wallace Venable