It will be extremely difficult since there has been aging on the old floor which causes a color change. You are going to have to purchase some new pieces of flooring and attempt to finish it to match, or at least come very close. Practice the staining on scrap wood until you achieve the look that you are looking for. You may even remove a few boards from an out-of-the way spot and install them in the closet space and place the newer boards in their place (such as along the wall under the bed or wherever).

Or in the back of a closet in another room