Regarding the specific m39 emr stock, you can try to match one instead of trying to find an original.....
You can get NSG stocks still on limited runs. Right now clyde armory has some in stock.
There are a few other things missing to go full EMR. Laser engraved rail number, stock extension, machined top rail, brown hand guard.
The buttstock extension (BEK) moves the rubber butt pad back about one inch. Those are only available in black right now as far as I know. I really like this extension as the cheekwweld felt too far forward on my jaw for me.
For the top cover you could get it machined yourself to fit a scope mount, or buy a second cover direct from Sage. Odds of them having a NSG one in stock though are slim.
I have no idea what would be involved in getting the BEK and a top rail to match NSG. I don't think its a simple cerakote job. A good shop might be able to at least get close enough on the extension.
The other thing is laser engraving on the rails to fully match. Sage does this as an added service if you send the stock to them.
Brown hand guards are no longer made. You will need to find a second hand one, or paint a black one. You could possibly cerakote one, but that is pretty iffy as the curing temp is near the melting point. I have been meanign to buy a spare and try this for science but haven't gotten around to it yet.