Tad just had a thread a few months ago where he repaired the same problem on an older Nacra 20. The bolts are welded to a plate and it's probably not reasonable to repair the plate - you should order a new one.

Once replaced, tension the dolphin striker as per the Nacra instructions. They usually have a measurement on the mast post where you put everything together, measure where the top of the beam is on the mast post, then start tensioning the big nut on the bottom of the beam until the beam raises so far on the post.

That said, that is a major problem. I would not sail it until that is repaired as it will be a catastrophic failure if that remaining bolt lets go.