You could remove the pan and look for it or you can be sure it didn't hang in the chain mechanism and forget it. If it makes it to the pan it won't hurt anything. It can not get sucked into the oil system as there is a screen on the oil pump pickup.
With the head off, you should be able to see enough of the gears to verify that the washer isn't there. Pull the cahin up and hold it while hand cranking the motor over. Hand crank it again after the head and chain are back together.
Bosch Master, ASE Master, L1
33 years MB technician