You could make copies of the door handle and horn grille. Take a small piece of clay or aluminum foil and take an impression of the existing pieces. Fill with 5-minute epoxy. When set up, pop the parts out, paint silver or foil them, and PRESTO! You have your missing parts!
Since the running boards are not identical, but right and left sides, you would have to scratchbuild the missing running board out of styrene sheet... but that shouldn't be too hard to do.