If the current bbls are totally unuseable, and the rest of the double is in perfect order. I would have new bbls installed using the existing mono block, ribs, sights etc. I have seen a couple of double rifles JJ has done this way, and if he did not mark the bbls "By JJ Peridou" you could not tell.
This way the rifle retains its original look, with brand new bbls of modern steel.

Just think an original British double with modern steel bbls regulated for the modern load.