Try the air and flexible magnet trick and you might get lucky and snag it. But remember that the wire fell into the gap, it's not like it is wedged in between the piston and the cylinder wall. The piece of wire also has very little mass and size so it's not going to be knocking holes in pistons, etc. I've seen perfectly good running engines that showed signs of ingesting stuff. There were dents in the top of pistons but no other issues. I doubt that a piece of wire brush wire is capable of denting a piston. On the other hand if you have only just bolted the head back on, pull it and retrieve the wire. You'll feel better and only be out the cost of a new gasket.
SimS
