I used to have a copy like that, too. I think I still do (somewhere). The bottom line is that they were relatively cheap to purchase (a couple of dollars or less) and had been a big sales item in the mid-1970's leading up the Bicentennial in 1976. Since it's a modern reprint, I think you can probably scout out replacing it for less cost than attempts at restoring the copy you have unless, of course, there is some family history or significance to the specific copy you have.
This is probably what you have. Still available for $4.95 (plus shipping) at this link:
http://www.libertybellshop.com/decl...rchment.html