DO NOT BUY THAT BOAT.
If it is truly a B20 motor, the circulating water pump is impossible to find.
A B20 is a overhead valve pushrod engine which is getting harder if not impossible to find parts.
The only way I would consider taking the boat is locally and for free with a test drive in the water.
You're thinking of the B30 inline 6 with the hard to find water pump. B20 is a 4 cylinder. Plenty of parts available if you know how to use a computer and the magical spell called google.
If you want to put the engine in a trophy room and compare against "modern technology", you aren't going to win any awards. If you want something that is easy and simple to work on and makes the boat move through the water, that engine does just fine.
However, it is a raw water cooled engine and may have suffered from corrosion if used in salt water. That's the reason I would be leery of it, and not because of the engine itself. There's also the issue of being a 44 year old boat. There's no way I would mess with it unless I was looking for a project.
I happen to have a boat built in 1976. Once it's in good shape, it's the same amount of maintenance as any other boat.