All great advice - I'd try Hawg's suggestion first.
The OP doesn't state what kind of barrel or muzzleloader it is.
Is it military? If so, does it have the typical 3 land/groove rifling? If so - polish it and try it out.
If it's a civilian style - just what is it and how deep is the rifling in it - also what caliber. I've polished some original barrels out and tried them - usually some pitting doesn't have that much effect on how well they shoot. As stated, "read" the patches.
Your question about "leading" the barrel is a good theory but I don't think that it is the answer. Leading will attract more leading - even with a patched ball sometimes. In theory, the patch keeps the ball from the bore but if it's rough and it cuts the patch, walla, it will add more lead.
Since the rifle was given to you and has some sentimental value, I wouldn't give up on it. Work on the existing bore and see what comes of it. First of all, since you don't mention what the rifle is, make sure that it is safe to shoot. If it is an antique - and the bore is in that bad of shape - the breech should be checked out by a competent gunsmith with experience in BP firearms. Antique barrels usually have a breechplug with a much coarser thread pitch than is used today - I've inspected some originals that if they were shot, the threads were in danger of giving way and nobody wants a breechplug between the yes - a bad way to die.
If it is shootable and you can't get the bore cleaned up to where it will shoot well - explore the possibility of having it "freshed out" to either a larger caliber (if the barrel wall thickness is thick enough) or having it sleeved. Two very good recommendations were made - Hoyt and Whitacre - they do outstanding work. There are others out there as well. It very well could be worth the price. Good luck and keep us posted on your progress.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63