Probably I should start by saying that I am a purist collector, and my views certainly do not align with everyone. To me, there is a very narrow window of what should be done, or not done, to an antique arm. Probably the cardinal rule is to do NOTHING irreversible. Second would be no refinishing, especially done by an amateur where the results could easily detract from, not enhance, the piece. In most cases, a light going-over with solvent and fine steel or brass wool for the metal, and a good rubdown with lemon oil or boiled linseed oil for the wood is all you want to do to a piece which you have been lucky enough to receive un-molested. If it has already been screwed over, the 'rules' are less rigid and more up to the individual. There is nothing wrong with replacing an entire part with one of MATCHING patina, color and wear, AND proper period, but tread carefully because the wrong part - such as a early hammer on a late gun - will stick out like a sore thumb. Yes, SA did upgrade parts but usually only sights. Interior lock parts, of course, are fair game to fix, but don't use an early coarse sear with a later three-notch tumbler or vice versa.
Relining would be fine for a cut-down rifle barrel, but I would not do it to an original carbine barrel. They may not be "rare" but they don't grow on trees, either. Two things to stay away from are Naval Jelly and ANY sort of power tool(s). If a prospective purchase bothers you to the point where you think use of such would help, lay it back down and save your money for a better specimen. We are only caretakers, and future generations will thank us for showing some restraint.