test to see if it's valves by putting about a tablespoon of oil in each cyl, then testing again. If that oil seals the rings and the compression goes up, it's the rings/pistons. You don't give numbers. How low? How many miles on the engine? Be warned: opening up these engines is opening a can of worms. There will be many things to do. I would (did) just go with a new engine. I suggest you read up on it, and do an estimate before proceeding.