I think what you need is in this pile of parts. Bidding is pretty low right now. You'll have to paste this link into your browser:
https://www.ebay.com/itm/ITHACA-LEFEVER-NITRO-SPECIAL-12-GAUGE-PARTS-LOT-SXS-DOUBLE/391976884114?hash=item5b43a49792:g:wJIAAOSwRTVae1TY
There is another Nitro Special parts lot on ebay that has 12 ga. hammers too, but it is more expensive. I see Nitro special parts there all the time. There are lots of other sources of used gun parts too, such as Numrich or Gun Garage. I think Nitro Special parts are far too common and inexpensive to go through the bother of making a hammer of the correct grade of steel and properly finishing and hardening it.
