If you can support the engine somehow, it might be possible to cut into the bedrails with a sawsall to get the broken bolt out. Then you can rebuild the bedrail with fiberglass &
epoxy. I think you'd still end up needing to drill down a new hole for the replacement mount, though, so maybe that won't work.
If the bolt is rusted, you actually have two problems: 1) where is the water coming from? Have you already fixed the source of the leak which allowed the water to attack the engine?, and 2) now that the water has rusted the engine bolt, it's also possible that it has penetrated the core material of your engine bed rails. It may be that they need to be rebuilt. Not to open a can of worms, but I would do some inspection and probing if I were you, before I started just trying to fix the one problem of the broken bolt in isolation.