How fast are you wanting to go? You don't need a motor swap of any kind. I have an '06 660 Rhino bored and stroked to a 780 with a ported head, 45 carb, dual exhaust, cam, piston, programmer, bronco clutch and it's more power than I need. It has a top end of close to 70, way faster than anyone needs to go in these things and has more low end grunt than anyone could need. Don't get a RZR unless you want to spend 50% of your time working on it because they break something every other time they go out it seems. As far as power, any machine can make enough if you use a good builder. My build is super reliable, monster torque and HP both. So go with the UTV you like the best and put some money into the motor.