What year is the boat?? The biggest problem you would have, is the out drive. It will need a gear ratio change at the least, if not a totally different unit to handle the horse power and torque. If you do install a bigger power plant with out changing the out drive to the rated motor, you could burn up the gears to the lack of cooling, shear the gears from over powering them, over heat the motor from the lack of water flow, if you don't have through hull exhaust, you can cook the out drive from exhaust heat and have a very restricted exhaust flow, etc... The list goes on and on. If you like, pm me with with some details and I can be more specific to you situation.