Before I would go to the trouble and expense of replacing the breech, I would use a buffer of cornmeal over the powder to lift the ball to the good section of the barrel. Let's try a less expensive approach first.
Well from what I saw, the pitting is involved over a long area..., and this is an odd type of construction, as the breech isn't a plug, it's the last few inches of the barrel. One might be able to have the ball sit above the pitting..., and good to try first, YES, but otherwise, this is the problem
LD