Ok well if you have no compression and the motor has overheated chances are you need a head gasket maybe even a head. a quick way to check to see if its the rings is to put a teaspoon of oil down one of the cylinders and do a compression check again and it will most likely do nothing because it sounds like its your head but that will clear out any chance of it being the rings. The only other thing i can think of you doing is to put some compressed air into a cylender and listen for where the air is leaking from. I hope that helps you, and good luck with it.