I would be cautious about removing seats and decks as they are usually part of the integral strength of the boat . If you remove anything make sure you add brackets or such to help keep the boat strength intact . For example bench seats that run the entire width of the boat are attached to the floor and both sides and help to strengthen the boat, if they are removed the sides of the boat are weakened, your modification should include a way to compensate .
Make sure you take a couple minutes and look for the rating plate in that boat, if it exists it should be back on or near the transom somewhere. I'd be very careful about hanging a 25 HP motor on a 12 foot boat, as it may not be rated for 25 HP. Some 12's are different than others, more depth, more beam, etc. so the motor ratings can vary.
Proceed with caution.