There are solutions if you can't find age appropriate parts.

Just about any machine shop could make a press-fit sleeve with internal threads that would accept some readily available BB. You could drill a hole (or 2) in the underside of the BB shell and weld the press fit sleeve after pressing into place for security. A bit of bondo and no one would know.

Or, you could thread the existing shell for "adaptor" cups that are threaded on the outside and inside.

Or, you could replace the shell with a different one seeing as the frame is lugged.

Or, you could just replace the frame with something that doesn't have all these problems.