IMO if you're patient you can find another one for less then what you pay to reline yours.
Investarms are great shooters with patched balls. My first .50cal is anyway, after I shimmed the adjustable rear sight. The sight blade was a little sloppy. I won provincials and a few trail walks with it.
5 years ago it was a 275 dollar investment with a mildly rough breech area, which cleaned up after about 100 balls.
And while you wait for a deal to show up, take the breech plug off and use a bronze brush with lots of penetrating oil. Clean the the rust out, and just shoot that thing. You may be surprised.
I have one of these as well. On the pictures it looked like a Green River Rifle Works hawken. But it was a fixed breach, bubba parts collection with a GRRW lock and trigger group. The barrel looked absolutely hopeless at first.
It was from a used gun site in Quebec.
It is still a parts collection, but it shoots good now. I have to scrub it after about 20 balls. Down the road I may use the lock and trigger for a proper Hawken build.
Cheers
ww270win