If your question pertains to the legal rating for the hull, then it's 130hp. The USCG requires boat manufacturers to list max hp on boats 20ft or less, purely for boater safety. However, most boaters believe this rating is stipulated by the manufacturer so to not exceed the integrity of the transom. This is not true. HP for boats 20' and less are governed by the USCG. The USCG law code is here, which includes the formula:
https://www.gpo.gov/fdsys/pkg/CFR-2002-title33-vol2/xml/CFR-2002-title33-vol2-sec183-53.xmlUsing the max hp rating formula, you'll be surprised to learn the actually rating for your boat is 150hp! A common procedure for boat manufacturers is to rate the boat for less than what the max hp the USCG would allow using the formula. The reason being boater safety, thus reducing the manufacturers risk of lawsuit.
Using Yamaha as an example, the dry weight difference between the 115hp/130hp and the 90hp is 100lbs. No weight difference between the 115 and 130. Which now brings me back to your question: which rating do I go by? The rating produced by the manufacturer, which is 130hp.
I have a Yamaha 2 stroke 115hp on my '76 170. My guess is the original designer of this hull intended the mounted motor dry weight to be less than 300lbs. Either that or engines 40 years ago were lighter per hp. The reason I state this is because when the boat is submerged at rest, the water line is above the location of the scuppers, regardless of the amount of fuel in the tank or any other weight factors. Since the hull is designed "self-bailing", technically the scuppers should be above the water line. No biggie, I just plug them when heading out.
Deciding on a 90hp, 115hp, or 130hp rests on 2 factors for you: your budget and how you intend to use the boat.
