You really don't have to many options here. If you disassemble the engine and look for a broken piece you might as well rebuild it. It's hard to tell where the metal piece came from, could be anything from dropping a washer thru the carb to wear of an internal engine part, to something that a previously rebuilder caused.
If it was me and the car still runs good, I would flush the engine with 4 qts of 10-40 and 1 pint of Marvel oil, let it idle for 15min. drain and refill with 10-30 and have fun driving it and have a Fosters! Sometimes we drive ourselves crazy over things we can't control...