As mooncoon suggested, making a single action hammer would be easier. The double action pawl could be omitted. The slot for the mainspring stirrup could be cut with a dremel disc.
To simplify manufacture even more, the hammer could be made from flat plate, and, if desired, the thumb spur could be made wider by silver brazing on a separate piece.
I would suggest making a thin sheet metal - or even cardboard - template for the hammer, working on the outside of the frame. Assemble the surviving parts externally, and it will be easier to determine the shape of the hammer. The actual hammer could have much of the shaping and fitting done with an external mockup. The mainspring stirrup should be made from tool steel, hardened and tempered. The hammer can be made from mild steel, and case hardened.