Your only allowed to edit the aircraft skin.
Actually the pilot files for all aircraft can be edited. I changed the skin tone of the black pilot on the 332nd Jug I did recently. For the P-51 its dude1.bmp and dude2.bmp. However I can't see that painting an oxygen mask on the face of the pilot is really going to work. It would need a new 3D shape to look any good and that can't be done.