I have a Marlin Mark I shotgun that is probably 40 or so years old. The plastic piece on the butt is warped/cracked and it is hard anyway. I'd like to find a nice soft rubber recoil pad for it if one is available. Any ideas? Thanks. Tom

I'd probably try a Sims grind-to-fit pad (or maybe even a slip-on).

Brownells.com will have many aftermarket pads.
For a oem replacement, if they have it, try Numrich Arms or Sarco. Just do a search on these las two.