Mesa Tactical did/does make some limited 20G stuff. I have MT +2 extension on mine, but appears they no longer have any extensions on their web page. Add Mesa Tactical to your search, maybe you'll find old stock somewhere.
Rem 20G tactical parts.... argh....
I inherited a 20G 870 project gun, that has been 8+ years of "almost complete" while trying to find compatible parts. The experience is what prevented my from buying a new Tac14 20G. Every part needs an adapter, and as they are low demand they are expensive/hard to find. Mesa Tactical has the Lucy 870 12G to 20G stock adapter which allowed me to finally replace the stock, but add a premium of $50+ just for the adapter.
Compatible fore-grips are rare as well. I have an aluminum ATI on mine, which I hate the look, but is very comfortable and has compatible rail mounts. Not sure what Remington/Magpul did with the Tac14 fore-grip, but nothing on Magpul site for 20G fore-grips, only 12G.
I have been looking for a 20G barrel/magazine clamp for 8 years, with no success.