Check with your local gunsmith. No one would be making new parts, but lots of the old double and single barrel shotguns were similar and he might have something laying around that he can make fit.
Neumann Freres was in business in Liege, Belgium from 1863 until 1957. Thousands of their double-barrel shotguns were imported to the US (marked with English "Bros." instead of French "Freres") prior...