If that's the original lock plate, it looks like it comes with some internal parts. Maybe correct, maybe not.
If it's a military gun, old original parts should be fairly easy to find to put the lock back to Flint configuration.
Taking care of the percussion drum threaded hole is another matter. Doing it right, making the work invisible, takes some skill and experience. An easy way is just saw the drum off, clean up the area, rust it up a bit and let it go at that. That way, if you gain more skill down the road, you can remove the threaded plug and do it up right.
As it is, you have a $300 gun. With a little work you can have a better looking gun worth a bit more. Or, if you mess it up trying stuff beyond your skill level, you can end up with a $100 mess.
John