Years back I had the chance to run one of those 12'-6" River Hawk boats with a 5hp Briggs Go-Devil Crawfish model. The hull design felt 'tipsy" with that mud motor and the speed was about max 8-10mph WOT with light load and 1 person. That boat seems like it needed more horsepower yet still needed to be lightly balanced in engine weight.
My suggestion is to Keep It Simple.
The Dixie motor is expensive for that boat and I don't think you gain much with the SD on that boat hull. Plus Dixie MM has some prior "customer relations" problems and they are currently in recovery mode on business practices, and quality control.
Stumpjumper looks OK, Subaru Robin motors are good, but it can be very hard to get hop-up parts for em'. So you may be stuck with mostly stock motor horsepower for a long time.
Maybe get a Stumpjumper frame and put a Honda GX200 (or GX 200 clone) on it? Easy to get about 9 horsepower with some simple race parts; and IMHO that River Hawk needs a lightweight 9hp.
Cheapest and way riskier
route for you is a fleabay SPS longtail mudmotor kit and modified Harbor Fright Predator 212cc clone ... about $800-900+ in price---but you need to be mechanically inclined and always vigilant about keeping that Oriental made stuff intact.Best o' Luck in your search!
