2008 polaris 770 compression issues

I have a 2008 Polaris 770, 700 with big bore kit, and last season I lost compression in one of my cylinders. Went down to almost no compression and sled barely moved then died. Would not keep an idle so towed it back to trailer. Took the head off and looked at the top of the pistons and cylinder walls. They looked clean with no shavings or anything. Moved the pistons up and down. Crank felt smooth and the cylinders looked fine on the top end. Put head back on and did a compression test. 125 and 4 pulls on each cylinder. Should I pull the jugs and look at the bottom end? Could it be the crank bearing? Stator? Any ideas or help would be greatly appreciated..

take the exhaust off and look at the piston. may have been something to do the the exhaust valve. my 2 cents