Unfortunately, the M16 stock have to remain OEM Systema.
RS M16 relies on the buffer tube as the stock platform. Adding a mil-spec buffer tube, mod a buffer+spacer+buffer spring to retain the cylinder assembly, and updating the mil-spec/variant stock will generate other problems...
Where to route the controller??? & where to place the 7.2V/9.6V/12V battery???
Oh yea... If a mil-spec stock is attached to the nominal Systema stock adaptor via a long bolt/bolt+adaptor+bolt, then envision a rifle stock that sags & wiggles like Jello gelatin...