This is a good problem to have! I have always maintained that the fun is in the search. If there is a capacity plate on your Richline, use that as a guide (in some states it’s more like a restriction) to the power you select. One of our members runs a Scott 40 on a 12′ tin runabout. One thing to consider is the strength of the hull. Some 12′ boats are not strong enough to carry more than 10 hp. I used to own a 12′ v bottom fishing boat rated for 7.5 hp but the transom would flex alarmingly when I ran a 5 on it so I greatly reinforced the transom and ran a Mark 20 on it. All that said, an overpowered boat will be squirrelly and unpredictable, perhaps dangerous. Sneak up on it, starting with the 15.