My 1436 flatbottom is rated for a 15 horse motor. I put a 13 horse mud motor on it. The boat weighs around 160 lbs, and the motor weighs around 150. There is no way I would want a heavier motor on the back. I'd say that it's at its limit.

Now with a 12 foot jon, a 9 horse would probably the largest you'd want. Top speed will probably be 8-10 mph with you and gear. Keep a paddle in your boat. They are good for maneuvering your boat in tight spots. It's also your reverse.