1963beretta - I think your information source is different than mine - I've only owned a Winchester-Cooey Model 840, not a Cooey 84 - go to Numrich - mostly showing "Out of Stock" for many of those parts, but not all - for Model 840 they list forearms for 28, 20, 16 and 12 gauge as same part number - but different part number for .410. Seems to be same part number listed for all butt stocks, though - they don't separate those by gauge - although they do separate out a "Youth" butt stock. Based on what I saw on the 840 that was here, as if they made the outside of the barrel on 28, 20, and 16 gauge the same size as 12 gauge - at least through the fore-arm area - much thicker walls on 28 gauge, then normally seen on most other shotguns, except for the .410, I presume. I guess I was assuming that Cooey did the same with their Model 84, before Winchester bought them.
ebrand - depending what you have for resources and skills, it would not be impossible to make replacements, especially if you already have originals and all the fittings for them.