Regarding the stock, wash it with soap and water, then liberally oil it with tung oil or linseed oil. Don't sand it, you will just screw it up that much more. For future information, for anyone that reads this and has an antique firearm. If the metal is heavily rusted use liberal amounts of 3 in 1 oil, and a piece of brass, and scrape the
oiled rust off using the brass. I use the edge of a fired 30-06 brass and an a piece of sheet brass for scrapers. This method will remove most of the rust and leave the
patina, which is everything when it comes to an antique firearm. The best advice is if you don't know, don't do it. I will look better browned, but in my opinion it won't
increase the value, just the looks. The octagon to round barrel, along with the length of the barrel, and the shape to the trigger guard makes me think it's a later cap lock era Indian Trade musket. Is the plate opposite the lock in the shape of a serpent or dragon? Your lock doesn't have the fox sitting in a circle which is another dead giveaway, so my swag is probably wrong, but the fox in a circle was common on trade musket locks, except is wasn't on all of them.