You should be able to use an 870 butt stock (may need mods). The 11-48 is recoil operated so the stock is more like the 870. The forearem...that is a different story. I have searched high and low for a synthetic forearm, with no success. You may be able to convert one from an 1100/87, but I have not tried.
I have a spare synthetic 870 stock I will try to put on my 11-48 tonight and post back.