Have you considered the various gasoline powered M series trucks from the early 1950s? M38, M37, M135/211 M35 series etc. as a parts source In basic appearance, their primer pumps are at least similar? The Canadian parts list for the M135 only lists the pump as an assembly (the cup isn't listed as an available service part).
Link to another forum discussing cleanup/maintenance of primer pumps -
http://www.willysmjeeps.com/v2/modul...ewtopic&t=5143