Kitten Formula
1 cup plain yoghurt preferably not the nonfat kind
1 envelope un-flavored gelatin
2 tablespoons Karo Syrup
1 can condensed milk
For an old cat you can add an essential fatty acid supplement. Show Winner brand at PetSmart does not have an offensive odor. Most smell like cod liver oil. This adds linoleic acid, linolenic acid and anachrondic (not the proper spelling) acid which are excellent for dry, flaky, itchy skin.
My cats would fight to get at this stuff when I was weaning kittens. It's rich because kittens are growing so fast but a little would get calories into an old cat without having to eat much. Even without it I would suggest the essential fatty acids sprinkled on regular food. It makes their coats full and shinny and probably reduces hair loss and hair balls.