Hal,
Maybe Dutchman Wood Works (www.dutchmanwoodworks.com )can help.I recall (from either "Der Waffenschmied"or "Waidmannsheil")that they replaced the missing forearm for another German Gun Collectors Assn. member.I happen to have a 12 ga underlever drilling with a cracked frame that I planed to sell for parts, but you wouldn't likely have a use for the rest of it.
Mike