As long as the boat is over 20', you are into unlimited horsepower. Now, the manufacturer may put a plate on your boat stating "recommended max" or "maximum" horsepower, but I believe this is more an attempt to limit their liability or warranty. My understanding is it is not a Coast Guard requirement on over 20' boats.
"There is a grey blur and a green blur. I try to stay on the grey one" Joey Dunlop, Isle of Man rider