I’m with Wayne, I’d try a fix. Solder as he said, heat from below. It’s worth a shot. Also, and this is just my feelings/opinion, so take it for what it’s worth, but we live in such a disposable world now that it seems like if anything isn’t “perfect”, we throw it out and buy new. Not saying that’s typically you, or us as a group, but a general reflection on our society today. In the 18th Century, that was very much not the case. Socks were darned, clothes patched. If a sword was broken, knives would be made from the blade.
To me, something like this is part of the journey, fun, and educational experience of 18th Century gun manufacture, especially in Colonial America. Like Mike pointed out, pin holes and small inclusions are present on a number of original rifles. Most aren’t to the extent in the posted photo, but who knows what was done by the original builder to “fix”, or mitigate the existing “flaw”.
Like I said, just my feelings on the topic, and certainly no criticism is intended. I’d give fixing it a try and see how it turns out. If it doesn’t work out, so be it, you can always purchase a new one, and you might pick up some new information, and have a little fun along the way. Best,
Ed