Preheat oven to 350°F. Coat a 9- x 5-inch loaf pan with cooking spray. Place buttermilk and oats in a bowl; stir to combine. Let stand 15 minutes. Whisk together pumpkin, maple syrup, oil, brown sugar, vanilla, and eggs in a large bowl. Stir in oat mixture. Set aside. Whisk together flour, pumpkin pie spice, baking powder, salt, and baking soda in a bowl. Add flour mixture to pumpkin mixture; whisk just until combined. Pour batter into prepared pan; sprinkle with pumpkin seed kernels. Bake at 350°F until a wooden pick inserted in center comes out clean, 55 to 65 minutes. Cool in pan on a wire rack 10 minutes. Remove bread from pan; cool completely. Cut into 12 slices.