If it's a connection to an internal layer, there's not really much you can do besides running a wire. I use kynar insulated wire-wrap wire, it's the same stuff that was occasionally used in the factory to repair traces or rework errors. It's not uncommon to see a few bits of it stuck on production boards from that era. PCBs were so expensive to build at the time that it was cheaper to rework them by hand than to scrap a faulty board.