You've got several options...If you can't find a factory barrel, Ed Rayle over in West Va will make you one to fit what you have...He can also rebore and re-rifle what you have...If it's a .50, go with a .54 and a 1-72 rifling...
Funny thing about these guns, they might look rough and still shoot OK...I had a friend bring me one that looked like that about 10 years ago, I cleaned it up, plugged the barrel and poured vinegar down it...Left it 2-3 days, poured it out and filled it up again for 2-3 days...I then cleaned it up and that rifle would shoot inside an inch and a half at 50 yards with patched round balls...My buddy's son in law still uses that gun today...
That barrel may very well do the same...In the 1700s, they also made smooth rifles...They were built like a rifle, had sights but no rifling...Plenty of guys hunt deer with a 20 gauge and a patched ball, they report good groups out to 75 yards or so...