Although I have never done it; I don't see any great difficulty fitting a breech plug when the barrel is already tapped and the plug is already threaded. With inlet black, a file, a wrench/extention and a vice I think one could do it quite easily if they had the patience.
Dovetails can be done as discribed already above. A brace or soldier gun would be needed as well. Stocks, locks, and furniture are pre-cast. I have some parts to make a drop-in flint barrel but I never get around to doing it; but it's not because I don't think I can do it.
I have a couple of GM barrels that swap out in a PA Hunter stock and an RMC fast twist flinter and I don't really need another but the key word here is need.
I don't know just how hard it would be to adapt a Lyman barrel to a TC or visa-versa but it could probably be done with too much work/expense. I like GM drop-in barrels but they don't have much for flinters which is the only way I go.