I did - and I'm close to saying it's not there. The coastguard's site links you to a 3rd party for the actual codified regs and laws. The link to the "Safe Powering" section is below.

http://law.justia.com/us/cfr/title33/33-2.0.1.8.44.html#33:2.0.1.8.44.4

The reg. over "Safe Powering" pertains to attaching a label indicating a higher max horsepower than is allowed under a prescribed formula - nothing about attaching a motor to the boat in excess of the prescribed maximum.

As I said. I've looked, I can't find it, what I've found doesn't preclude installing an oversized motor. It seems odd that the section on Safe Powering wouldn't actually address safe powering, so I'm hoping someone knowledgeable can chime in with the actual material