1. Preheat oven to 400°.
2. To prepare streusel, combine first 5 ingredients in a small bowl with a fork until mixture resembles small peas.
3. To prepare muffins, place 18 paper muffin cup liners in muffin cups. Coat liners with cooking spray; set aside.
4. Weigh or lightly spoon brown rice flour, potato starch, and tapioca flour into dry measuring cups; level with a knife. Combine brown rice flour, potato starch, tapioca flour, granulated sugar, and next 4 ingredients in a large bowl, stirring with a whisk. Stir in raisins and walnuts; make a well in center of mixture. Combine oil, vanilla, and eggs in a medium bowl, stirring with a whisk. Stir in soy milk; add to rice flour mixture, stirring just until moist. Spoon batter evenly into prepared muffin cups. Sprinkle batter evenly with streusel topping.
5. Bake at 400° for 20 minutes or until muffins are lightly browned and spring back when touched lightly in center. Cool in pans 5 minutes on a wire rack. Serve warm.