Jack, I have done similar thing with an old 14ft jon boat which I sold 7-8 years ago. Then I did a similar thing with an old 16ft tri-hull bass boat which I still keep out at the pond. If it were me I would mount the trolling motor up front and make a rudder for the back, you would be suprised how much better the boat will handle with a rudder, fishing out of the front will be much easier too. I made the homemade rudder out of an old trolling motor I found at a garage sale, I will see about getting a picture and posting it to give you some ideas.

One other thing you can do with the plywood is to seal the edges so it does not delaminate due to water exposure over time. You can mix a two part epoxy and then thin it down with laquer thinner, then use and old brush to aply it to the edges of the plywood. The thinned epoxy mixture will then soak into the edges for a very nice seal that will last a very long time.