There really isnt anything stopping you putting a larger size hp engine on the boat but your insurance will be high, however if in the event of a claim i wouldnt be too sure wether this would cause a problem?? I suppose a lot would depend on what make boat we are talking about, if its a very popular one like a fletcher then the insurance company would probably question the size engine, but if it was one of my Seaquels then they probably would be none the wiser as to what Kw rating the boat has unless they contacted the manufacturer or they already insured a number of them previously.
Tricky subject really!
The other thing to consider is that ratings are set at what they are according to the lay up of the craft so if a manufactuerer rates at 70Kw and you put 85Kw and something happens, you are the one left responsible,,,? However these ratings are very much over the top anyway to cover the RCD notified body!