I have some spare 12 ga forends. They might be too big, but can be modified. I have no idea what they came from. One has ejectors. What type of lug is on the barrel? Could you post a photo? One of mine might be close enough to have a decent smith fit it.
There were Rigby latches, Deeley and Edge pull down, simple wedges on early guns, a "sugar-tong" double spring system, Anson push button and many others. So, we cannot drill down to a solution until we know what latching system your gun uses. The forend lug is the give away. Being BLNE makes it easier, no ejector bits to fool with.
We'll also need a photo of the front of the reciever to see what the cocking arms look like.
Joe