Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x5-inch loaf pan.
Mix soy milk with lemon juice in a small bowl, and allow to stand for 10 minutes. Mix together the flour, oats, brown sugar, baking powder, baking soda, salt, and 1/4 teaspoon of cinnamon. Mix together the soured soy milk, plums, 1/2 cup walnuts, soy yogurt, and vegetable oil, and pour the wet ingredients into the flour mixture. Stir until just moistened, and place the batter into the prepared loaf pan. Mix together raw sugar, 2 tablespoons of walnuts, and 1/4 teaspoon of cinnamon in a bowl, and sprinkle over the batter.
Bake in the preheated oven until a toothpick inserted into the center of the loaf comes out clean, about 45 minutes.
