Is the boat in a Salt water environment? I would think that if it had any exposure to salt air it would show signs of corrosion, or at least discoloring. My boat was in salt water 20 years ago, been in fresh water since but the engine still shows areas when the
paint is removed. Engine has been painted a couple of times. Other areas of the boat show the same tell-tale green from the SALT water environment. If you are really concerned a engine survey should satisfy you of it's present condition.;
Good Luck,
John