The regulations(Coast Guard??) state that you can't put a motor over 90hp on boats under 18 ft.. Lots of them running around out there, but I'd be willing to bet it wasn't done at the factory.

On another note...you will find that the same 90hp motor will push an 18 foot boat better than a 16 foot boat. The downside of the boat wieghing an extra 150 lbs. is less than the upside of having 12 square feet more planing surface on the bottom of the boat. It is easier to get the boat on plain, and push it across the water when the weight of the boat is dispersed that much more. Keep the motor, buy a little bigger boat. If you don't believe me, go drive one if you can find one rigged the same...Good luck in your purchase....