I guess I'm giong to play Devil's advocate today...
What if the prev. owner had the boat inspected by the USCG and had the boat approved for a bigger (up to 150hp) motor on the thing? Then I would say he is legal, as long as he can prove, in writing, that the USCG said it was safe. He may have an aftermarket (non factory) sticker on the boat stating the HP is good up to 150, after all, he is close to the Ocean, and may need/want the extra HP. I sure as hell know that I would want the most HP possible for an ocean going vessel. Plus, there may have been some type of option from the factory for an Salt water package, that maybe allowed the stepped up HP rating?

These are all un knowns, until the guy can actuall look at the boat.
Normally I would say that the current owner is full of horse dung, because I've never seen a 22 footer that old, allow a 150hp motor either, hell my 2005 25 footer is only rated for 125hp.