That's a pisser!
I bet a shop vac on the intake and compressed air through the spark plug hole would get most if not all of it.
And I'd check/suck them all out before you try to turn the motor over.
That one may be visible as a mouse pantry because the valve is closed. How did he figure out which one would hold? by trying them all I expect.
Once you have vacuumed them all, then slowly spin the motor while continuing to vac out the cylinders.
A pain to be sure, but I doubt there's anything in there hard enough to do a lot of dammage to the motor.
Good luck!