Yeah, that's definitely a stress fracture at the back of that pin. I'll talk to the metallurgist I work with for his suggestion on the best steel to use. Other than that I'd figure 4140 heat treated to RC45. The firing pin on a 1911 is specified as medium high to high carbon steel between 40 and 46 Rockwell. If it doesn't hold up at least we tried.
CNC produced 416 stainless triggers to replace the plastic triggers on Colt Mustangs, Mustang Plus II's, MK IV Government .380's and Sig P238's and P938's. Plus Colt Mustang hardened 416 guide rods, and Llama .32 and .380 recoil spring buttons, checkered nicely and blued.