I'll throw my 2 cents worth in, in regards to the "lock". As you probably know, the two lock bolts that go through the "side plate" on your "42 need to be removed so that the lock can be removed from the "lock mortise". The internals will consist of the sear, sear spring (small spring at the rear of the lock plate), bridle, tumbler and mainspring. Once you look at it, you'll quickly see the function of each individual part.
My main intent is to warn you in regards to removing the mainspring. This would be the first internal part to remove. For this, yo will need a "main spring vise". You can get these through different suppliers - Track of the Wolf, Dixie, etc. A mainspring vice is designed to compress the spring so that it can be removed from the lock plate - the main spring vise has a "bar" which puts pressure on the lower leg of the mainspring along the length of it. Yea, yea . . . you'll hear of some folks substituting a "C clamp" and it can be done, BUT, it's kind of like using a hammer for a screwdriver. You run the risk of snapping the mainspring because pressure is not applied equally on the bottom portion of the spring. I've seen a number of main springs broken in this manner over the years. If you have a muzzleloader with a lock, you need to have a main spring vise in your tool box. As they say . . "the right tool for the right job". They aren't that expensive and you'll get your use out of it each time you disassemble a lock.
Your '42 sounds like a great present! As you say, you can also get a modern made barrel to drop in it for shooting if need be. Whitacre and others make excellent barrels . . . check the NSSA site for links.
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