Agree, bigger motor a bad plan. You should really have a look at the weight of the current motor and anything you might be considering. 50 lbs might not seem like a big deal - until you see what it does to the back of the boat, with it being much lower in the back. This can get to the point the back of the boat is easily swamped. To be considered as well is the design of the engine pod, the part the motor hangs on. On some of the older stuff, they were built when a 40 hp motor was seen there more often than not.....
You don't mention how old the boat is, but my guess is the pontoon diameter is quite a bit smaller than what you're going to find on the newest stuff (25"+). They just won't provide the necessary flotation a bigger motor requires. Upside to the older 'toons is they ride noticeably better in the chop....as long as they are properly loaded anyway.
