Don't waste your time and money trying to do it. You will end up with a unstable platform totally worthless for fishing or anything else. Part of a skis stability is the ballast the motor provides deep in the center of the keel. An outboard strapped to the back of a ski hull will just unbalance the whole mess further. You will spend all your time trying to keep it upright. Plus you don't have to go through the hassle of trying to title a "home-built" boat. I don't know about your state, but it is a real PITA in mine.
Why not just buy a used jon boat and put your outboard on that. You can find them for under $300 (probably less than you would have to put into modifying a ski), and it will give you a much better fishing platform.