Preheat oven to 350 degrees F. Coat a 9-inch springform pan with cooking spray.
To prepare cake: Toss raisins and rum in a small bowl and let stand. Whisk flour, nutmeg and salt in another bowl.
Mash sweet potato in a large bowl (you should have about 1 3/4 cups). Add eggs; beat with an electric mixer on medium speed until combined. Add coconut milk, 1 cup brown sugar and butter; beat until combined. Stir in the dry ingredients until evenly moistened. Stir in the raisins and any remaining rum. Spread the batter in the prepared pan.
To prepare topping: Combine coconut, 2 tablespoons brown sugar and cinnamon in a small bowl. Sprinkle on top of the cake.
Bake the cake until a knife inserted into the center comes out clean, 1 to 1 1/4 hours. Let cool in the pan for 10 minutes. Run a knife around the edge of the pan and gently remove the side ring. Let cool at room temperature for 1 hour, then refrigerate until cold, about 3 hours.