The gun is a Model 1842, the first U.S. percussion musket. It is .69, and was made as a smoothbore, but many were rifled in the Civil War era. They were not considered first line weapons, but were often used in combat nevertheless.
I don't think you will find a restorer who will consider working on that gun, it is just too far gone, and a true restoration would cost more than the gun would be worth. I would clean it of active rust, dirt and dust, and hang it on the wall.
You might replace the foreend tip if you can find one at a reasonable price (try
www.ssfirearms.com) and put in a repro cleaning rod (same place).
The Model 1842 is a fairly common gun. Harpers Ferry made about 100,000 of them and Springfield another 170,000. Decent ones are going for around $1600. Relics like yours bring a couple of hundred.
Jim