for brown sugar: granulated white sugar mixed with molasses (or maple syrup for a different flavor) at ratio of approximately 1 cup sugar to 1 tablespoon of molasses (which is basically what store bought brown sugar is from my understanding) ...or just plain white granulated sugar without the molasses.
cornmeal is a bit harder to do without a change in flavor: corn grits/flour would be close to the flavor since they are also made from corn but ground to a different degree which can change the texture in the end food, but bread crumbs or other types of flour (e.g. wheat, rice) can also be used, but are likely to have a more noticeable impact on the flavor even though they might be of similar texture. In all cases it may require some tweaks to amount used in the recipe.
