In my mind, (a very confused place), I see any boat that has been altered, as not having a specific horsepower rating. If it has been altered for your personal use based on your engineering, I don't see how the original Coast Guard hp rating could come into play as far as making your boat legal or not. I say do what you think is right, and test it a bit, then test it hard, and determine for yourself whether you have done the right thing.
An example comes to mind. My neighbor and I once shared a fence, and a love for boating, and a mutual friend who built boats. I bought a ten footer from our mutual friend, rated at fifteen horse, and did mount a 15 horse. My neighbor, thinking a little outside the box, looked upon this ten footer as more of a jet ski, and despite the CG 15hp rating, put on a 40 hp Yamaha 2 stroke, and drove from a console mounted amidship. The boat ran around 50 mph, with a fun factor of ten plus. I always wished I had HIS boat. :-)
Master of Disaster