John wrote:
I don't know if this engine is old enough to have been derated by the
use of a thicker head gasket to compensate for poor quality fuel. So,
you may not get the 90 HP you're expecting.

If it hasn't been rebuilt yet, it's a ticking time bomb. Factor in the
cost of a rebuilt power head ($1800 - $2000) when negotiating final
price. The good news is, if it lasted this long, the previous owner
took care of it.

You don't know that! It may have low hours, and been maintained very
well. The year that it was made has nothing to do with anything!