It's very common for small pieces of aluminum to fall out of the head casting between the valve seat inserts and usually doesn't mean anything.
It rarely means that the head is cracked.
Pressure test them. If the test shows that you have a leak then the seats need to come out and the crack welded up.
Aluminum welds and repairs well BUT the issue is when you go to press the new seats in. You need to freeze them first.
Probably they should be pressure tested again after the new seats are pressed in.
You can't reuse the old seats. You need to install new ones.
Those are probably going to be Chevy cast iron seats? Be thankful.
My aluminum 427 GT40 heads used stellite seats. Those no longer existed plus I don't know how you cut them anyway since they are harder then the stones.