The briggs and stratton air cooled motor is a great choice...a little loud though...but it wont freeze up on you. As far as horse power for your Jon Boat, most of these HP ratings are based on width. Your 10 foot jon would go like stink with a 15 hp on a stright away, and the transom could probably handle it, but once you crank the motor to make a sharp turn water could come over the side. Keeping that in mind, go with what you are comfortable with and your driving habits. I have a 14' with a 15 hp, but my marsh has very narrow cuts where turning isnt an option. I did have a 30hp on it but the weight of carrying the motor to and from the boat wasnt worth the extra HP.
"teal, teal, albino teal....wait, those are seagulls...take em.
wait till they line up and cross, 1 shell 2 birds