You did well resurrecting this easy fixer. Are you sure you want to pull it all apart just to paint the base? It looks quite good to me. A clean would suffice I reckon. There is some rust visible so it may be worth it I'd you are wanting to eliminate all the rust spots.
How did you figure out that our needed the diaphragm and gasket replaced?
Could you tell me where the diaphragm is located?
I have a Victa with Chonda motor that is giving me problems and I might try replacing those if I can get it as cheap as you did.

Seems that mower shop is typical of many these days. Focused on selling more than servicing.
My local one is the same. You walk in and if feels like a showroom more than a mower shop.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!