I have a Polaris 4X4 Explorer 400...I was told that having the bottom half of the piston broken off doesn't effect speed or take off. The pistons itself it slightly loose. Would it be the carb or the piston causing me problems?? The thing runs...idles kind of rough...but theres just no power on take off and tops out at 40mph or so(usually running up to 60mph or more). I have the thing in pieces right now...getting everything back in shape. I'd rather not put it back together if I need to overhaul the engine. Advice would be much appreciated.

Get the sleeve honed and get a new piston. It's cheap $200 total and a lot cheaper that a new motor.
As for the bottom of the piston being broke, it will allow for the fuel to enter the cylinder befor it is suppost to