I was going to say the same thing about dropping a valve. My sisters '91 CRX had an exaust valve snap and fall into the cylinder creating almost exactly what your #6 cylinder shows. Her piston was cracked, exaust valve was gone, intake valve was cracked with pieces missing, and those valve chunks smashed the heck out of the spark plug. All these pieces from that one cylinder traveled from the #1 cylinder to #2, #3, and #4 cylinders, with less destruction the further away from #1 you got. I didn't know that debris could travel through an engine like that. The whole engine was toast and I had to replace it for her. She dosen't have good luck with cars. She recently bought a '94 Camry and 2 weeks later, #1 cylinder was toast. A simple test showed that the compression rings on the piston were shot. Opened up the engine a few days ago and found out the cylinder wall is gouged. Now I have to replace the engine in her Camry too . So much for getting work done on the Z
Anyways, the point I was trying to make is look closely at the head that you want to reuse. Use some parts cleaner and get that carbon off of there to see what the rest of the head looks like. There may not be damage to just your #6. Also, debris can get caught between the valve and the valve seat, so check that also.
If this were my engine, it would be a smart move to just get a junkyard head to throw on there, instead of the smashed up one you have now.
!M!