Make a new set of half nuts! OF course you can only do this if you have a another lathe available and a mill. If there is enough cast iron left on the casting, you have two choices of what you can do. One, is to make a bronze nut to bolt into the half nut halves. I guess you could build up with bronze welding rod, machine new thread afterwards. Second is get some Moglice putty and form a new set of threads using the existing lead screw as a pattern.
Last thing you can do is get ahold of Logan Actuator Co, and visit with Scott Logan about a new set of half nuts. Don't be shocked at the price for new half nuts!