Just a thought on this, however. There are motor control units that will electronically adjust the speed of a motor - they're often sold for routers but there is no reason they won't work on other motors. If you bought one of these to use on your antique scroll saw, you could adjust the pulleys for the maximum speed you'll ever used and then slow down the motor electronically to the speed you want.
I tried this on my AMT...It doesn't work unless it is a motor with brushes... the variable speed controler started smoking pretty bad