I agree with Albert. Grind off the face of the frizzen, and maybe drill a hole at the termination of the crack, to stop the crack from running all way across the frizzen. Braze or silver solder a shoe on the frizzen. The hole drilled to stop the crack from running can be used to pin the shoe to the frizzen for brazing.
For that matter, I have seen a coupla originals that have the shoe pinned near the top of the frizzen and two pins at the bottom. However, in this case, I suspect brazing or hard silver soldering would be the best bet.
Track has a coupla blank frizzens sold to reconvert old locks. Might check the size of those.
Do you have another lock with a frizzen that will fit? Sometimes things work out easily, sometimes not.
If all else fails Jerry Huddleston mentioned making frizzens from heavy angle iron. Never done that, so can't offer anything in that area, but support.