Safe to run and drive?

Yeah, it is only one bolt missing. But is a very important bolt. If the bolt above it, or the one further up was missing, I wouldn't worry about it.
The engine & trans are 'suspended' on their respective mounts. All of that combined weight is taken by the BH bolts. The weight is trying to close any gap at the top of the BH, but it is trying to spread the gap at the two bottom BH bolts.
What would worry me would be if the car hit a big bump or pothole, anything that unsettled that weight & pitched it around. A proper brace would fix it but the brace in the pic is just strap iron & would likely allow too much flex, the thing that is trying to be contained.
If it was my car I would get some round steel bar 3/4 to 1" diam, about 3/4" long. Drill it to take the bottom BH bolt. Cut the existing brace & weld it to the side of the steel bar.

If you don't have the ability to do a repair it'll probably be just fine. If you can though, cut the ear off the dead engine, do a matching cut on the new engine and weld the ear on. Easy repair if you have access to the tools.

If your paying for that engine I would just get another candidate, I personally wouldn’t waste time on something like that unless it’s already in my car. There’s a reason why a huge bolt goes in there.