This is the way I have done repairs when the pieces were not available to glue back in.
Take out the glass, remove the screws holding the motor unit , let it dangle on the wires if there is no connector handy , tape some oiled/greased , grease proof paper over the outside of the hole , use some fibre glass resin to blend in and fill the hole from the inside allow to harden remove the tape and repeat the process on the out side, blend in the contours of the mirror case by adding layers of body filler, sand to a smooth finish , blending into the surrounding paint work cover with primer, smooth with 1500/2000 grade wet and dry abrasive paper , repeat until smooth finish is obtained , spray with final colour until you are satisfied with final finish , use abrasive between coats if you get runs or am not happy with the finish as is.