What's wrong with this motor?
 

When you crank the motor with no plug, carb or exhaust it sounds fine. I try to check compression and it sounds like something is smashing around inside and the flywheel looks like it stops turning. I tried uploading a video, but it says it isn't an acceptable type? Gonna end up pulling it apart just want to know what I'm looking for.

On most ATVs the starter motor drives a one way clutch behind the flywheel. If this slips, the starter doesn't turn the engine and it can make quite a noise.

How do I replace the one way clutch? Is it just remove flywheel and it's behind or do I have to go in from the other side of motor? Thanks

On any I have come across you remove the flywheel and it either comes off with it, or is left sat on the crankshaft behind the flywheel. Some have gear teeth round the outside which mesh with the starter gears, some have a chain to starter, which, of course, has to come off before the one way clutch can be removed.

Thanks to Merryman for nailing this one. I ended up just swapping a flywheel/one way clutch assembly from a parts motor and everything is working perfectly now.