I'll take the bad news first

 

Bad news is tough access. Good news is that not that uncommon situation.

I also have a 1988 Mazda truck and had its engine rebuilt professionally in 2020. About 3 weeks later the engine blew a freeze plug out, was repaired under warranty and no issues since with that or the other freeze plugs. They used a metal freeze plug to fix/match the others, but I remember seeing rubber expandable freeze plugs available. No issues since then.

 

What I'm thinking of doing is stuffing cloth right under the hole and then drop a rubber expandable plug in there and tighten it up.