The single port pump rebuildable cores have probably mostly been worn out, rebuilt, worn out again, and trashed for quite some time, while spares for the heavy duty pumps have survived. Remember that it's been over 50 years since any of these engine were produced. Try finding rebuildable spares for anything related to Model A's or Model T's these days- - - - -they used to be hard to give away! Ditto for flathead V8 stuff!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!