I posted a while back about my sisters GTP having a rough idle, which turned out to be a smashed spark plug. Well, it ran fine for a little while, then promptly ate the same spark plug 2 more times. My old man had a spark plug lying around with the same thread that was a little shorter, so that seems to have gotten the car running again, but apparently it's got a knock in it now. I looked the plugs over and they're all gouged up, which is pretty convincing evidence that there's metal of some kind flying around inside the cylinder. I'm thinking probably a ring/ring land gave way, but there's no way to know for certain what exactly happened without pulling the head. What is for certain is that the engine is toast, there's no possibility of this being something minor anymore. So my question is, what should we expect to pay for a replacement engine, and what years interchange?