Freeze plug out, now in, what else?

[Sail365]

hello,
I'm looking at purchasing a boat that has what seems to be a sound hull. There are plenty of items that need addressing but i think all reasonable, and in that the price is right. We took her for a sea trial and the engine ran hot. Skip and I looked at her and found that a freeze plug on the atomic universal *kabota block as i understand it) had let loose. Clearly, even in a southern state that rarely freezes and the water is always >45 degF, had frozen. With the freeze plug replaced she starts instantly, doesnt smoke, and runs cool. I would otherwise buy this boat, but what is the risk of hidden block crack or other issues, and how would i figure this out, in an otherwise strong running engine? Much obliged for the input!

[Bill Seal]

From Wikipedia:

The term Freeze plug is slang, the name of the press in block plugs is actually core plug. It is mistakenly thought that the purpose of these plugs is to be pushed out and save the block from cracking if the engine has water in it and it happens to freeze. This is nothing more than an urban legend or an old wives tale. The purpose of the plugs is to fill the holes that were made during the casting process, so the foundry could remove the core sand from the coolant passages. Saving the block from cracking in case of a freeze was never the manufacturer's intent for these plugs

[a64pilot]

Freeze plugs failing are often an indicator of an improperly maintained cooling system, they fail usually from corrosion. Likely a good cleaning flush is in order.
I assume this is a fresh water cooled engine?