Bart,

I would not even think about cleaning or polishing the weapon. Just leave it alone. Trying to "spruce it up" for whatever reason will inevitably diminish its value. Collectors like the piece to have a patina to show how the gun's wood and metal have aged naturally.

If you want, you could try getting a replacement nipple at Lodgewood Manufacturing. http://www.lodgewood.com/
The likely have repro nipples and may have some original ones as well, for not too much.

However, a cracked nipple or cone is not a big deal in my opinion ... unless you're considering live firing it, which I would advise against without first taking it to gunsmith for an expert opinion. One could argue that the cracked cone shows the age of the piece as well, even if it was damaged from dry firing years after the war. Some would say it's part of the character of the gun. Some would say to replace it if possible. Guess it's up to you. I'd probably leave it be, unless I was sure I could find an original cone for the piece whose patina c;osely matched the rifle musket.

Mike