I just don't see the demand and profit margin for it.
I've had potential customers ask me about making original stocks (wood, natch), and my answer was, why? When you can buy a stack of them from AIM for fifty bucks as mentioned above.
It might be what you
want, but I can assure you there's not enough demand to justify the tooling to reproduce the original stock in plastic.
I am curious, though, as to what's driving your desire for this. The original military stocks have lousy ergonomics...and well, pretty much butt ugly.
Why would you want one in plastic? Just askin....