I have used old vacuum cleaner motors to power several organs, lowering
the speed (and noise) with small variable transformers, available for
a few dollars by catalog orders from surplus suppliers.  This is much
more efficient (and cooler) than using a resistor and you can adjust
the voltage by trial to the exact need.  Several such small variable
transformers are offered on eBay right now.

Dale Gunnar
Corpus Christi, Texas