I think those tiny bits of copper are no danger to the engine. No more than carbon can be as they are much softer. Maybe only 1/8" in length.Seems as though it would be a good time to pull the head off and remove whatever it is in cylinders 5 & 7.
If one of those pieces does get stuck under an exhaust valve when it's in the car and all assembled, then it's way more work.
I'd suggest just buying the bolt kit or grabbing bolts from a yard for the exhaust manifolds.