Defender and Jamestown Distributors are both dependable sources if you are nearby. I'm not so I saved a bit recently replacing a cracked diaphragm plate (WHA-AS0561) for our manual bilge pump (an original Whale Henderson Mk 5) using JMS Online. Your part, same source: Whale Deckplate Gaiter Kit WHA-AS3725.

Replacing the trim ring several years ago, the mounting holes didn't line up so I had to drill and chamfer new ones.