I wouldn't fret on that minor volume. You can easily overcome that by milling .007 off the heads, in terms of compression ratio. I have never seen evidence of detonation around that "ring". Healthy engines will build carbon there, and it helps to self-heal the space. Not trying to be critical, but your engine seems to have a huge oil problem.
Since your pic shows it with great clarity, check out that little tab on that 1020 gasket that's hanging over in the water hole. That tab is responsible for rusty decks when using 1020s. It is just enough to seep water between the gasket and block, and it traps there and chews on your block until the next teardown. 1020s work good when they don't do that. Some blocks don't have that situation, some do....
A nicely honed cylinder and some better rings, tight valve guides, and Viton seals will dry that baby up. Also pull your pcv hose.......if it's full of oil, re-examine your baffling. If that hose is dry, look closer at you intake gasket seal on the bottom......intake fit, etc....just my two cents......