Basically the last 8' of a 40" Jon, with the transom cut out unless absolutely needed for saftey. Of course at that point just buy a Jon to begin...
What about flipping her over, filling the chanels with wood, then attach a plywood "wing" with no edges effectively widening the stern of the boat...
I have a Coleman Scanoe as well. I use it for river fishing and power it with a 5hp outboard. I can get up to 11 mph but would like to go...