5ph is the max rating for a 13' Riverhawk...granted guys run boats overpowered all the time, but running a boat at the max USCG rating isn't running underpowered in my opinion...whether the boat would be overloaded as described (2 adults?, dog, cooler, gear)...I guess it depends how big you guys and the dog are, but you might be pushing it...not calling you fat either, just saying if it were me (200lbs) my older son (100lbs) my dog (90lbs) we are already pushing it and haven't even added in battery, gas, cooler and gear...
Here is a link to the Gheenoe website showing 2 adults in a 13' running with a 5hp:
http://gheenoe.net/thirteenft.htmlI'm guessing that all they have in that boat are the 2 people, the motor and just enough gas to film the video clip

But still should give you a point of reference as the 13' Gheenoe is essentially the same hull design as the 13' Riverhawk...the Riverhawk hull is actually 20lbs lighter than the Gheenoe variety.
Anyway, hope this helps...not trying to be argumentative, but I still think 5hp and 8-12mph is enough juice to safely navigate the Caney under Generation. Nothing beats taking the boat for a test drive though...have the seller meet you at the Caney and take it for a spin if at all possible.
Good Luck,
Dave