If you have enough to finish the quilt for a double bed, I would complete that part. I would then add 2 or 3 borders to get the needed 12 more inches for a queen. You will probably never be able to match the original close enough to not be noticable. There are product designed for fixing old fabrics but I think I might wait until the top is completed. I can't remember the name but if you type in old fabric stain in the search something may come up. The subject has been discussed before on QB.