I agree with Bobbielinks....I would finish it, quilt it and then worry about getting the yellowing out. It might not come out, but the quilt will still be valuable and cherished. Otherwise, if you decide to try to clean it now, use some clear or very thin paper and trace out the quilting that is on the fabric. Make sure you have some type registration marks so that you can match the placement. Also, take a bunch of pictures to help you remember.
I found this interesting article about different ways to remove the yellowing.
http://www.home-ec101.com/how-to-cle...intage-linens/