its next to impossible to mod an outboard and have it become a rewarding expierence theres not alot of wiggle room especially in the smaller ones theyre pretty much maxed out per cubic inch from the factory best thing i can tell youif you want more power is to sell this one and try to find a bigger one
if it were me id shoot for an older model than this and sacrifice newness for power and price ive seen ALOT of older (80's models) 15hp johnny rudes in the $600 range lately and its just a personal preference of mine but i wouldnt own an engine made after 1985 they just arent built as well and are really tough to work on and if i cant work on it myself i dont want to own it.
my 1956 fastwin has 100psi in both cylinders, weighs about 67lbs, and has more grunt than any 15hp motor made today oh and it only cost me 20 bux i probably have about 150 in parts to get her going and thats a complete tear down, new coils, complete water pump kit, tune up kit, lower unit gaskets, driveshaft (to date that was the most expensive part $40) carb kit, and a fuel bowl(i broke it getting it off the carb and its glass)
keep an eye out
if you were to mod it youd probably only gain 2-3hp MAX and youd have ALOT of $$$$ in it to do so (read machine work and porting)and youd ruin the resale value
its a nice motor and i think youd be able to get more than what youve got in it and find something a little older for less than what youd get out of that motor
Im in the same situation as you. I have a 8 hp johnson and was wanting more power for my highsider. I think I will soon switch to the older 15 2 stroke motors for the weight and overall power. My buddies NMZ scoots with a 15 and 2 people. Just make sure the transom is nice and solid or reinforced.