Bevel the edges of the broken piece and the block. Then the piece can be welded back with cast iron rod, high nickel rod or braze it in. Use a small C clamp to hold it in place while welding. Use a bolt to initially keep the threads lined up. Use caution on how mich heat is put in to reduce cracking at the edge of any weld. Weld 1/4" to 3/8" and then gently tap to stress relieve the weld. When done and it is cooled, run a tap down to clean the threads.
