I can undertake the work to reverse engineer what's missing, the forend is very simple, the gun is an extractor not an ejector, and it's of a back action design but not a dominion action the lock plates fit up entirely differently. It's marked but .450 ex so I think that would be BPE rather than nitro so it's an earlier gun

I've made new forend irons up before when they have gone missing it's not a big job particularly in this case Where the only moving part is the extractor cam, the difficulty is that the design could be a lever type catch or a push rod.

As for the lockwork there is nothing wholely difficult about the lockwork, there will be a lot of time in milling the lock plate as it has to come out of very thick stock in order to include all the features. I can draw the inverted form of the RHS lock work and make that.. it's just going to require making a fair bit if tooling for a one off.

The mainspring I'm still at a loss on there is no stirrup I can only presume that some how the springs had roller bearings which acted directly on the back of the hammers