I'd solder the hammer pieces together, find a piece of flat stock of the same thickness, drill a hole of the proper size for the hammer screw in the stock, locate the soldered hammer on the stock with the hammer screw and a clamp, draw the outline, cut it out with a hack or band saw and bring it to final shape with files.  Round off the top surface, checker the spur, caseharden it and stone the full-cock notch for a decent trigger pull.

The fly in the 44-1/2s is generally missing on the single-trigger examples I can afford, and no problems have ever been noticed.  It's needed for the double set trigger to keep the hammer from snagging in the half-cock on the way down, but a pull on the single trigger moves the trigger sear well in front of the hammer sear, too far to catch.

This hammer has the fly in it ,so actually there are 3 pieces.   It is split so the fly fits in. Does it matter if you leave the fly out?