Code of Federal Regulations Title 33, part 183 specifies that a boat under 20 feet in length have a capacity plate--this includes an outboard motor HP capacity, if an outboard is to be fitted.
I suspect that any law enforcement officer could deem a boat manifestly unsafe if it is overloaded or has too large a motor, this seems to be rarely enforced.
The C Dory 22 is over 20 feet and does not fall under these regulations. However each manufacture sets a size of motor based on transom design, materials and hull design. One could get in problems with their insurance company if these were exceeded. Also there would be a legal issue if an injury or damage occured because of too large a motor.
I believe that the origional C Dory 22's were rated for 90 hp, then 100, and I have seen some with 115. I understand that the factory will not certify an older boat for a larger motor than it was designed for at the build time.
Many people put larger motors on their boats--and for the most part get away with it--but, the C Dory is a type of hull which does not lend to high speeds. I felt that our C Dory 22, with a 90 hp was near its upper limit in the low 30's--the steering becomes skitterish at ligher speeds and not as positive. Some of the older boats may really not have the transom strength to take the higher hp or weight of the newer motors.
I have a 115 Yahama on a 18 foot CC Century--and it is a great engine. I really like the Suzuki's on the Tom Cat. Biggest probelm is that you cannot hear them at an idle. When fueling up, the gas dock attendant told me to be sure and start my engines, as I was shoving off--they had been running for about a minute and he could not hear them. I agree all of the engines are good in todays world. But decide if there is really a reason for more than 90 hp. The 90 hp will push the boat into the 30's even with a load for two weeks of cruising--and that is as fast as you need to go in this boat.