If this boat is brand new and you got such a screaming deal on it, I would first try to make every effort possible to get it back to the owner it was stolen from, and if that's impossible, I would then turn around and sell it for $750 and buy a boat (even if it's used) that fits you better.
If you use the boat, it is similar to you using size 150cm skis. You will seriously be limiting your ability to progress in the sport, performance will be affected. That size boat is definitely for people much smaller than you. While the boat might be fun to "throw around" in flat water, once you enter any kind of moving whitewater, you will really find out how much of a mistake this boat was.
Putting it on the water even once and scratching it up will affect it's resale value much like driving a car off the lot. But being able to sell the boat "brand new" (of course, only after all efforts to find the victim of the theft) will maximize it's value and your profit.