Wash and dry sweet potato. Wrap well in foil and roast in oven or toaster oven for 1 hour at 400F. It should be quite soft and easily pricked by a fork. If it's not soft enough, roast another 15 minutes. Let cool. Peel and mash. You should have about 1 cup of mashed sweet potato, give or take a little. This step can be done ahead of time.
Mix topping ingredients together with fork or fingers, until it is a crumbly mixture. Wrap and place in fridge until ready to use. This can also be done ahead of time.
Preheat oven to 350F. In large bowl, combine flours, flax, cinnamon, nutmeg, baking soda, baking powder, and salt. Mix well. In another large bowl, combine eggs, sugars, sweet potato, oil, vanilla, and water.
Gently fold wet ingredients into the dry ingredients, just until completely moistened. Do not over stir. Spoon batter into lined muffin pan, filling to the tops. Sprinkle crumb topping generously over muffins. Bake for 19-21 min, or until toothpick comes out almost clean. It is done when toothpick still has a few tender crumbs sticking to it. Carefully test it to make sure muffins are not over baked. Cool muffins in pan about 15 minutes. Remove muffins onto wire rack. Enjoy!
Notes
If you want to make a bread loaf instead of muffins, just pour batter into a greased/floured 9x5 inch loaf pan. Sprinkle topping over loaf. Bake at 350F for 50-55 minutes or until toothpick shows a few tender crumbs.