2004 Chevy Impala Top Engine knock

I have a 2004 Impala that has a bad sound coming from the top of the motor. I recently had the rod bearings replaced but its still making a clacking sound like its out of oil (i know its not). Some people say its a stuck hydraulic valve lifter. Is it OK to drive like that? Could it be the cam or crank? Rod bearings were completely gone but I still have a knock. They were thin all the way to the bottom metal to metal. I was told that was the problem, but obviously its not. If the cam bearings are bad, whats the general cost to fix that? Or should I replace the motor? The car is paid off. How long would it take to fix?