For BMX riding I'm not sure if a shim is a good idea. And it's true that a 0.083 wall might be too thin for the same reasons. If you can find a guy with a lathe for the threads (I would not recommend cutting threads from scratch using a die) then you could get a thicker tube (say 0.120 wall) and have him machine the OD for the stem (tapering the transition would be best vs. a square shoulder).
What's the issue with the fork?