Bottom bracket conversion with rare cups
Depends on


How much you value originality,
The thickness of the metal at the cup threads
The internal diameter of the cup hole.


Either keep it totally original, which means scouring ebay for old bike parts (expensive)

OR look at retapping the BB shell to accept a modern cartridge bearing unit.  This is probably beyond the home mechanic because it will require a large tap and a way to keep it completely straight.  A frame builder might be your best contact.   A BB cartridge is cheap, common, and easy to change IF the threads match, and the width is workable.

OR see if there's some combination of press-fit BB that will fit in the BB shell.  I'm not familiar with these, but there was one where two parts entered and threadded themselves together in the middle, allowing a hollowtech axle to go though.  Would increase Q factor and might look a bit weird.

OR your proposed hack should work just as well on the Raleigh as on your bike.  Downside is if it needs servicing and you're not available to do it.

This is an interesting question - please consider posting your own answer, ideally with progress pictures.