You'll find the service manual, , useful.
I have repaired questionable bellows by bonding darkroom blackout cloth to the inside of the bellows using DAP Weldwood Nonflammable Contact Cement, . Darkroom blackout cloth may not be available but player piano bellows cloth is, rubberized .008 thick cotton or nylon. Cut the cloth to be the same length of the bellows and an inch or two wider than the perimeter of the bellows. Offsetting the seam of the new lining apply the contact cement to about half of one side of the bellows, apply the cloth cloth side to the cement, press out any air bubbles and repeat for the adjoining area until the bellows is completed. Allow to dry. The dried contact cement rubs off easily. Refold the bellows one pleat at a time. Very durable.