Good afternoon. Just finished rebuilding a 1996 engine. After cleaning up the shop, I found a piece of screen the shape of a kidney about 1” long. Should ‘‘tis have been installed in the head?  Thanks

Until you're ready to take the head cover off again
You might consider taking a hunk of a stainless scrubby and stuffing it in the hose that goes from head to air cleaner, and doing that in a way that doesn't plug it off, and make sure it's running uphill as far down the length as possible.
The idea of it is to let the oil vapors slap into something and be scavenged out of the air, and drip back into the engine.

You goofed, but it's not fatal and with care, you can work around it .
If oil consumption doesn't demand a quick solution, you might go with it until you have a reason to be back in there.