What would the highest amount of hp that you could put onto a 12ft tinny?

I would have to say that it depends on the material the boat is made from. I have a little 12' fishing tinny at my cottage that was ped down from my grandfather to my father and now to me. The boat weighed less than 100lbs and is easily carried up a steep hill to it's winter restin by two people. The biggest motor we could get on the boat was a 15hp 2-stroke Evinrude or a 9.9hp 4-stroke Mercury. Anything more than that and the boat would have flipped over backward when unloaded. During the summers I work at a marina and regularly see people with much newer and much heavier 12 and 14 footers hang 25 and 35hp motors off the back. Having dragged these boats around at the beginning and end of the season, I can tell you that these boats are at least double the weight of my old tinny (and somewhat better constructed) and are easily able to hold the weight of the bigger motors. Go for a bigger motor if you can and remember that 2-strokes are significantly lighter than 4-strokes of the same power.