What parts on your current Stag aren't milspec?
barrel, BCG, receiver extension, buffer, hammer, etc.?
Anyway, I agree with bejay, this is mainly a $100 upgrade to a new rifle where Stag uses a barrel and BCG that meet military specs. Replacing the barrel on an existing AR and the other parts would cost much more than $100 unless you could resell the leftovers for a good price.