The only part that doesn't fit exactly is a replacement forend. The Hawk slide tube is slightly larger in dia than an 870.
For a mag extension, a +1 from Wilson or Vang Comp is the way to go. A +2 from any mfg will extend past the muzzle.
For a sling mount, the Wilson can be ordered with a sling plate.
Don't know about a recoil pad, but assume a Limbsaver for an 870 would fit. As far as I know, the Hawk pad is better than a regular 870 pad.
For a light, a LaserLyte tri-rail and a Streamlight TRL-1 is the way to go.