What is causing overheating and leaking coolant on 2001 Polaris 425 sportsman ATV?

Check your oil for anti-freeze , check the anti-freeze for oil . Check the tailpipe to see if anything is coming out . If there is no visable leaks , its possible that the head gasket is blown pushing compression into the cooling system , pushing the coolant out . Check the filler assembly at the radiator , to see if it has gotten hot and warped .