Dang! - I misplaced my flywheel nut on my 56 evinrude 30hp - which I am guessing is the same as you have,
And recall that the size you mentioned MAY be it - but honestly - all I can recall is that it was a fine thread pattern and that the hardware store had a replacement for less than a dollar. Ended up finding the original - which had a very slightly thinner profile.
Good Luck