I have another 1.6 mech 11mm head in the garage. I'll pull it off the engine it's on and dress it up on the bench. Perhaps even port it.
Headstuds. Good investment.
Also a good time to check the valve clearances and order proper pucks.
YOU BUGGER, All this discussion and you had one off the car anyway. Well the ideas were pretty good to get out of us just the same.
It does sound like air in the IP. Bad valve seals wouldn't likely cause it to run badly, just sooty for a bit.
One leaky return line could let air in, when sitting.