my grandma has a 90 hp boat on a 15 ft fiberglass boat, which usually gets about 25mph so ur doing good compared to my grandmas boat
somethings wrong there. YOu should be doing low to mid 40's.
As for the merc, congrats.
That 2 stroke probably coulda been fixed up with a simple carburetor clean, decarbonizing, and new indexed plugs.