I see two options. 1. Drill additional holes in the windshield frame to align with the mounting tabs on the motors and install nutserts to accept a small screw. 2. Fabricate an extension tab to one or the other side of the bracket on the motor to elongate it for better alignment with the holes already in the frame. This is the option I would be pursuing first, were I in your shoes. I dislike drilling new holes into vintage sheet metal.