I would say get a new head gasket from thatsdax (under warranty?) and install it. Any metal "shards" (which are very soft aluminum anyways) that came off that and you didn't find are probably long gone.
If you are really worried, Remove the engine from the bike, pull the head and jug, and rinse it (the bottom end/cases) out with kerosine or diesel, then dump it out and repete until you are satisfied ther are not more "shards" in the case. It shouldn't require a complete disassembly.