No reproduction wheels available. Is the rim rusted through? The drums and brake backing plates are heavy steel, and can usually be saved by a good sand blasting job.
Spokes are available, so that is not an issue.
Are these the early narrow rims or the late style 2" wide rim? If you are not worried about originality, Worksman makes a very heavy duty bicycle rim that can be laced into the simplex hubs and work really well.