An FYI - It can be done but the juice isn't likely worth the squeeze for most.
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You can buy most of the parts for an MP5 A3 stock off of Robert RTGParts website.  (I've always gotten lots of good stuff from there over the years.) and its mostly NOS German.  He didn't have any MP5 stock rails but has stock rails for a HK93 - although a little long you could cut them to the same size as MP5.  I like the 93 length as I am taller.

The parts you must find on your own is the unobtainable  torsion spring and butt section - Since this was just a stub for a Tailhook, I Did mold my own out of quick-steel and it looks just okay.

Some mods are required as the cut outs on the 93 rails are not finished for a 94 had to mill out a few cut-outs.  Without the torsion spring, You can manually manipulate the locking mechanism but it won't spring open and lock  Might make my own spring!

I painted with Walmart $1 gloss black and primer - Actually the paint doesn't look like a bad match to the POF pistol

For $250  and hours into it - its not generally worth it - but I didn't want to butcher a viable A3 stock to brace.

For $295 I have a stock coming from an individual on HKPRO.com and that will go on after SBR paperwork is approved.  But I am okay with how this came out for me to use - It's not a factory stock that was ruined and keeps its pistol status for now.

Looks good to me. Nice work