Older Eljers used the PF/2: http://www.guillens.com/pf2/Default.htm
Newer ones use a Sloan
Either of these can have the flush cartride and the supply group replaced when necessry. If it is a Sloan, take the complete model number information off the black tank with you to the store. Parts also readily available from friend Google.
Now, here is a man's toilet!!!: http://www.flushmate.com/Supercharged%20Poster.pdf