I know it's in the boaters handbook not to do this (attaching a motor greater in size than recommended by the manufacturer) - but is that strictly a safety recommendation (ie unenforceable) or is there an actual law against it?

Hmmm...makes me wonder then what was on a pontoon boat I recently saw. Mine gets up to a max 15 mph. This pontoon boat we saw was going at least 25-30...my daughter was quick to ask how come we don't go that fast when you tow me on the tube lol.

What law? Code sec/ statute etc?

Not trying to come across as a jerk, but too often people think they know something but never look it up to be sure what they believe is actually true and go about their business blindly assuming their uncorroborated belief is valid.

Especially when it comes to laws.

I research legalities every day at work, I'm quite adept at it - but I'm well aware of the truth to the old saying "You don't know what you don't know" and I just because I haven't found it doesn't mean it doesn't exist. But anecdotes are not going to answer my question.

You could also ask Skip in the DEP Section I know that I have been checked in CT and also in Maine to see if I was within limits of the tag

As far as a pre 60s boat with no tag I would say run what you feel is safe for you and the boat Nothing for DEP to reference it to But that is just my 2 cents