Make the cake: Preheat the oven to 350 degrees F. Brush an 8-inch square baking pan with canola oil. Beat the egg whites and granulated sugar with a mixer until stiff peaks form, about 8 minutes.
Meanwhile, whisk both flours, the almonds and baking powder in a bowl. Beat the egg yolks in a large bowl. Whisk 1/2 cup oil, the orange and lemon juices and the marmalade into the yolks until combined. Stir in the dry ingredients, then fold in the beaten egg whites until just combined. Add the raisins, if desired.
Transfer the batter to the prepared pan. Bake until the cake is golden and springs back when touched, about 30 minutes.
Meanwhile, make the syrup: Bring 3/4 cup water and the granulated sugar to a boil in a saucepan; squeeze in the orange juice and add the peel. Simmer until syrupy, about 20 minutes. Cool slightly.
Pour the syrup over the cake while both are still warm. Cool completely, then cover with plastic wrap and let soak overnight. Cut into squares and top with confectioners' sugar and/or almonds.