Yeah I'd avoid running this engine any further and first endoscope the cylinder to see if that piece is still in there; if so attempt extraction and inspect for damage to every surface inside the cylinder. If no obvious serious damage then compression test. If you're lucky it eroded over time (could be a couple issues) or if it was a solid chunk off somehow got shot out the exhaust before extensive damage. If you won the lottery there then test that cylinder's injector. If the injector is good then it could've been that plug was loose.