I guess the question will be for you now D is if you change the barrel in any shape or form then you lose the originality & perhaps the collector's value of the rifle.
If that being the case 1500 quid to do it up & in the same calibre - back to where you were originally trying to find odd sized projectiles?
Is there any chance of relining?, this will still be a whole lot of money to resurrect the old girl!
Go the whole hog & do the Cinderella treatment, bring it back to a more easily obtained cartridge/components that will still work through your action & was possibly available in this rifle?
or
don't & spend on the money on another project.

I guess it will all come to us one day when a barrel wears out in a antique rifle.
I have 2 P14's at home, now each 100 years old; both have wear in the barrels but still shootable.
I don't envy you mate what-so-ever! I guess if I had to make the choice I'd try to keep original calibre & reuse sights, redo markings & engravings.
They will all wear out & being military rifles the barrels were replaced at times & proof marked as such!
Will be interesting whichever way you go mate!