Soft water is advised over hard water, but generally the amounts of ions involved is chicken feed. The choices of antifreeze are generally between glycol and alcohols. Glycols need buffering, due to possible acidity developing with age, and alcohols tend to be volatile, so evaporate.
Biggest problem, I suspect, is likely that of combustion gases entering the cooling system and causing corrosion, these days.
First question might be does the machine reside in a frost zone? If not, a buffered water coolant is likely perfectly satisfactory - remember these engines are cast iron, not of mixed-metal construction (piston material does not count). Glycol antifreeze is the more popular choice but is very toxic. I most certainly would not be ‘sucked-in’ to purchasing specialised modern engine coolant solutions for a middle of the last century technology engine.