The ailerons were fabric covered. Cut out the damaged part, leaving a ragged edge, and superglue some strips of thin cloth (maybe fine silk or rayon, something with a very tight weave) in the hole, then shred it and paint the whole thing the same color as the wing. Put a piece of very fine wire (I sometimes heat up sprue and stretch it) in the trailing edge of the control surface to simulate the frame. Bending it a bit would add realism, too.
Be creative, blacken it a bit to look like a 20mm shell exploded on the surface.