I have looked through these websites so far, but have found nothing.

Summit Racing https://www.summitracing.com/about/retail
Powerbrake https://www.powerbrake-drivetrain.com/master-cylinder-wagner-ff54846-ohk/
Partsgeek https://www.partsgeek.com/
Autozone https://www.autozone.com/
Fleettruckparts https://www.fleettruckparts.com/search?Make=GMC-Chevy
LMCTruck
Rockauto
NAPA

Maybe another wheel brake cylinder will fit.

The Delco Moraine is attached with three screws and the holes for the lines sit centrally.
I have found a similar one, but this is only attached with two screws.

Drum brake wheel cylinder
Model Delco Moraine 5467726
1 1/2 inch

The money for the parts is not the problem, is hopefully a one-time thing. The shipping is also unique.
Maybe someone has an insider tip or knows if there is a complete conversion kit.

Greeting Brian

1970 Chevrolet C10 Stepside and 1966 Chevrolet C60 Firetruck (Germany)