Why not make an entirely now "body" for the gun ? The mag housing is just sheet metal, not <too> hard to work with, the "body" is a piece of pipe, and a mag well could reasonable be constructed out of either sheet metal, or even a piece of square tubing for that matter, just put in a <tiny> sliver of metal at the far end of the newly contructed mag well so "original" sten mags wouldn't fit without modification (cut the back end of one feed lips by one mm or 2 so it wouldn't cath on the above mentioned sliver). Then make a new selector that doesn't have the FA position, and is again maybe a fraction of a mm smaller so an original wouldn't fit. There you go, a sten that uses <some> original innards but all the "offending" parts are now newly manufactured and don't fit the original. Think of it as an AR that doesn't have, and can't have without modification, the auto sear installed, while using (or being compatible with) all the other bits. It <should> pass inspection, as you've now complied with all the requested requirements.