Rich, I think you'll find it very difficult indeed to source an IT filter box, its a bit of a tough one really, to be honest although maybe not ideal I'd stick with the K&N , maybe buy a new one of those or something like it, are you sure the slide is blackened? some slides were black from new ...what number is on this carb, and what diameter is it where the airbox boot would fasten? Has the bike got a working exhaust fitted to this frame then?
Post up some pictures of the bike