When you say 'cleaning up' this gun, do mean trying to get it to work? Restore it so it looks nice?
Please be very very careful, so many guns have been ruined because they were cleaned up. Unless you are a proffessional restorer, you more than likely will ruin any value this gun has by simply removing some rust.
Please, find out what it is worth before you do anything further to the gun. I would hate to see, and I bet you would feel bad, if you accidentally ruin a piece of American history.