At work I make a popular spelt muffin that is sugar and fat free (well one egg in there) using crushed pineapple and yogourt. Cinnamon or other spices gives it flavour and a mixture of fresh and dried fruits help with moisture and taste.
You can't "just substitute" yoghurt. You would have to develop (or find) a recipe that uses a different method to arrive at the same result. I don;t have any recipes but you'll be better off finding a specific recipe that';s been tested that has the same characteristics you want.