The trigger group and stock are welded together. Where they mate to the receiver is apparently slightly different too, preventing the easy use of standard trigger groups, or stocks.
Also there is a modification to the mag well so they will only take MKE's 10 round mags.
IMO If you really want the rifle config you would be better off getting a Cohare/SW gun as they are much easier to work on, and take standard HK parts. A few are available new right now on GB.