Any chance you can post a photo of it? It is possible to weld cast iron but it takes a long time to preheat the area.
There is also a product called "Belzona" that is commonly used in the paper mill industry. They use it to build up metal shafts when the bearings went out and chewed them up. That can save them a million dollars per day in lost production when they don't have to wait for a new shaft from overseas. The product is mixed and applied like epoxy, but once cured, it can be machined on a lathe just like metal. You would mold it around the bolt threads and onto the block. A special mold release agent is used on the bolt threads so the bolt can be removed again. The stuff is real expensive but it works well.