The DivAir has a replacement diaphragm available from Jeff Heaton in BC. He is busy and makes them one at a time by order. See Vintagescubasupply.com for a link to his email address. He also makes custom hoses. Several people have had him make hoses for their DivAirs. I find the EPDM superstretch 1" hoses which are available through some of the vintage parts sellers a good match for the originals, except for the color.
One thing that can hang you up is a replacement nylon plunger, which is not available anywhere I know of. They are similar to, but bigger than the Healthways plungers. They can be resurfaced if necessary.
DO NOT try to take the yoke nut off, if it is stuck!!! You can break the fiberglass body (if that's the model you have). You can get the valve parts out w/o taking it off. Feel free to PM me if you need more information.