I had the same type of boat, we traded it in for our 06 Fisher. It too had some rivet issues (loose). we filled the bottom of the boat with water in the garage and did a perimeter check to see were the water was coming out from. It turned out that the water was coming out from right under the motor. We removed the motor, then one person put a heavy peice of iron on the inside of the boat while i took a sledge hammer to the outside, a couple of shots and the leak was no more! Ours was a Profisherman 17' it had a 80hp mercury. That was plenty of power, you shouldnt need to upgrade the motor on that boat, it will do 30-35mph no prob and thats plenty.
Hope this helped you,
Jason