Preheat oven to 400 degrees. Line a 9-inch pie pan with half of the pie pastry. Bake pie shell according to instructions (for fruit pie). Set aside.
For the filling, combine mango, sugar, cornstarch, lemon juice, rum and cinnamon in a bowl; stir until the sugar has dissolved. Set aside.
For topping, combine all the ingredients except the butter (or margarine) in a large mixing bowl. Cut butter into mixture with a pastry blender until mixture resembles coarse bread crumbs.
Transfer mango mixture into the prepared pie pan, then layer with the topping.
Bake in the preheated oven until the topping is golden brown and the juices are bubbling, about 45 minutes.