I am facing the same situation. This is how I think I may approach the problem and I'd be happy to hear some feedback on this.
1. Locate and trace the correct oval dent shape in the proper place of the helmet.
2. drill a small starting point in the helmet to allow the insertion of a fine tooth saw blade.
3. Saw out the shape and remove the piece.
4. Sand and fill the piece until smooth on the reverse side.
5. the piece can then be flipped upside down and should be close to the correct concave indentation shape when placed in the same
spot from inside the helmet.
6. Use epoxy resin to adhere it in place.
7. Fill in with 3M spot putty filler, sand the hard edge until rounded over.
8. Mask off the dome only, Prime and re-paint or Rub and Buff the dome.