I'll stick my neck out and say that IIRC, you can replace that firing pin without taking the gun down, but you have to be able to hold the breechblock in a vise to do it; that may require another pair of hands, plus brass or copper vise jaws.
But a very common problem with those guns is that so many firing pins are home made after the original broke, and were made too long. Then the rifle was dry fired and the long firing pin battered up the end of the chamber so the cartridge rim is not supported and the gun won't fire. I think you need to take a good look at that gun, or have someone knowledgeable do so to make sure what is wrong and if it can be fixed. You also need to determine if the gun is worth fixing; they are all old and many are just plain worn out.
Jim
