Magpul SGA stock and MOE forend is a very good set for Remington 870 shotgun.
You can replace standard buttpad with softer Remington Supercell or Limbsaver buttpad. Choose buttpad for synthetic Remington 870 stock.
Mapul SGA OEM Butt Pad Adapter comes without buttpadd and screws. You can find the screw at any local hardware store: 1″ long , # 2 phillips wood screw.
Limbsaver Recoil Pad for Knoxx Stock Review: should you use it?
I like the Knoxx SpecOps stock because it reduces recoil as advertised. But I have always wondered if it is possible to reduce recoil even more. This weekend I have tested Knoxx SpecOps stock with Limbsaver Recoil Pad. I haven’t noticed much difference. So, I think it is not necessary to change the standard Knoxx recoil pad.