For off road use (neglecting motard here), whether and engine pumps out a peak of 12HP, 14HP or 16HP. I think peak power ratings (real or not) are of very limited value. Sure you need a certain amount of HP, but high peak HP and nothing below that is just slow. What is both fast and FUN for off road and in particular MX track type riding is a broad spread of torque. After all, these chinese engines are effectively 3 speeds (1st is a joker granny gear), so with a wide spread of torque, you can sprocket size your final drive with a taller gearing, and get a very good rear wheel usable torque spread in each gear (2nd to 4th). This equates to real easy control (via gear choice and throttle) of rear wheel output for turns, jumps or whatever, which is fast and a lot of FUN.
So if you a targeting dirt riding, then you have some choices before buying an engine, 1) Test ride one. 2) See what the people racing are using (engine type and what mods). 3) Rely on opinions and get as many as you can.