Maybe???
First of all, Unity went out of business was what I heard a few years back, Jackie went from being a good friend and always helpful to shrill and manic and then at some point I heard they closed. If not I apologize but pretty sure they are gone.
I am not sure I can help with EXACTLY what you need but might get you *close*
I have a variety of NotRun-Norton mount plates for Singles and twins (As well as a LOT of Nortons) and can trace them for you. (Trans plates as well) I also have preunit Triumph as well, And I believe a complete collection of preunit twins and singles BSA plates for a variety of frames (Rigid-plunger and swing arm) but I have not verified them all, But happy to provide tracings of them as well.
It would be up to you to meld the BSA-Norton plates together for a hybrid-custom. Not hard to do, but time consuming (Start with file folder paper, cardboard, then transfer to thin wood or Hardy board and test fit repeatedly then make your own plates out of steel or alloy.
I can post photos too..
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I am in need/desire for similar for Preunit Triumph twins using A) Triumph Trans B) BSA Trans into Swing arm BSA A10 type frames. I will eventually make my own if I cant find any, but thought I would throw it out there myself.