Louis,
I had a 20ft Privateer Roamer 2000, that was rated for 200hp. The boat had a Johnson 225 on the transom. When I questioned the builder, I was informed the transom could support the extra weight, and torque.
I insured through BoatUS, not an eyebrow was raised when they asked the size of boat and horsepower of engine. I received my insurance paperwork year after year without a problem.
Marine Police didn't care as long as I didn't throw a wake in a no wake zone, and the Coasties never bothered as I wasn't faster than they were!
If its the boat you want, I wouldn't let 20hp stand in your way!