This is a wild and almost certainly ridiculous Hail Mary suggestion, but: A British outfit (like Holts Auctioneers, for example) may have a complete, clapped-out, off-face and rattly specimen on offer for a couple of hundred pounds. They also may have a junk drawer of sorts. If I were really interested in getting the thing usable again, and considering the cost of replicating a complete forend from whole cloth, I'd make a few enquires to some of the more voluminous British sellers, asking them if they'd let me pay for the shotgun but only ship me the forend or its iron. That'd possibly save you the bizzare and expensive ritual of importing the entire shotgun.