I won't address the legal aspect of your question. Here are some thoughts on the rest of the situation.
Weight - The added weight of the heavier motor will affect how much load you should carry and where you can put it. I'd be careful about allowing people to move around much, especially if they move aft.
Motor pod / transom - The extra downward weight of the motor, the twisting effect of the propeller thrust trying to pull the top of the transom aft, and the overall forward push of the motor on the transom, engine pod and boat structure will be greater when the engine is operated at higher power settings. If the motor pod / transom was not reinforced or changed by the previous owner, I'd be hesitant to operate the boat at anything more than about 80% power.
Handling - If you solve the above problems you will still be operating a relatively short pontoon boat with a heavy, powerful engine on the back. I can't tell you how much different it would be from using the rated engine, but if it were mine, I'd take some time without many people on board to learn how it reacts to rapid changes in speed and direction. And know that it will change some if you are in rough water conditions.
Bottom line: If it were mine, I'd reinforce the engine pod / transom and go easy with the throttle until I got very comfortable with its handling traits.
I often wonder why people ask questions like this AFTER the purchase?
Agree on the structural and flotation issues. Structure will be hard to spot, but the flotation might be a little easier. If the pontoons are floating more than about half submerged at rest you might consider moving some weight toward the front of the boat.
I've never seen an engine weight rating. Is that in company literature or something? If that's true (rated for 450 lbs) that should do it for sure! At that kind of weight, seems like you're covered for about anything available.
I know the Tracker factory installs a lot of the engines for Tracker boats. Do you know who installed this engine?