Seems like one could drill out the end of the shaft AND the inside of the nut simultaneously, taking great care to find the perfect center-punch point to drill. Thread the nut to the new inside diameter (lotsa meat around the hole in the nut), find a (rusty) bolt with the new diameter and threads, then weld the new/old shaft to the old one well inside the wooden handle, thinking hard about the desired finished length. Maybe some old, dirty grease buffed into the "new" rusted bolt-end would reproduce the appropriate patina? (Sorry, I'm big on invisible repair/restoration when possible and I promise not to tell.) Nice wrench, by the way, and best wishes.