I would think that you might first wish to have a gunsmith familiar with English fine guns have a look at it first, to make sure the barrels are good, on face, tight, and in proof (or have enough thickness remaining to endure a rebore and remain in proof), and ensure that the locks, ejectors, are in good internal condition. The cost of proper mechanical repairs to return an old best gun to service can fast outrun the cost of simple restocking. Nothing on these guns is cheap.... But they are such a pleasure to own, and shoot, they are well worth the cost!
Good luck!