Sunl 90cc
 

I just bought a used 90cc Sunl atv for my little guy from someone that didn't want to work on it anymore got it home drained the gas and pulled the carb and cleaned it....the thing started right up and runs great but after it get warmed up it struggles to get up hills....the engine bogs down really low and doesn't move much at all....it does fine if you have a running start for the hill but if you stop and start to go again it won't pull it....what do I need to check or replace? Clutch?

Just from your description sound like the belt or clutches if the engine runs ok.

Smooth
 

The engine runs really smooth and does great on flat ground and slight hills but bigger hills and more then a slight hill after warmed up just just bogs the engine down real low

Under a load a stretched belt can do this,worn clutch parts also. If it goes up a hill with a running start,but doesn't starting out at the bottom,then I'd still suspect a belt or clutch problem or possibly both. You can tell pretty quick on most models when you pull the clutch cover and check the belt. It should be tight.If it's loose or burnt or glazed on the edges a simple belt replacement may be all that's needed?