howdy, I'm pretty sure its a regular nut and not left handed, as for increasing horsepower the first place to go is the muffler IMO. The stock mufflers are somewhat restricted. Your 25cc has more hp than the 27cc bc yours is off of the "pro series" equipment. Are you running an adjustable Walbro carb or the Zama junk? Walbro wt227-1 or wt264-1 would be a possible upgrade if its not adjustable that way you can really dial in your prop I've never seen one converted for a boat but sounds cool. Oh almost forgot if its a used trimmer engine ck the exhaust port when u get the muffler off and clean out the built up carbon. What size "prop" will this thing use?