Contact the manufacturer but I'd bet it is doubtful they have replacement covering. If they did, it would probably be as expensive as complete replacement. Could talk to a siding installer who wraps windows, etc that could form up some metal. Maybe cheaper but not much.
Have you thought of sanding and painting? If the coverings are rusted, etc, your baseboard heaters may have a buildup inside from the water which would shorten the life. Your plumber could check that out. Good luck.