I'm gonna go with carbon build-up on #6 exhaust valve. The coil pack was weak or the plug was bad, causing poor combustion, hence the fouled plug and carbon build-up. Don't make the classic mistake of condemning the engine before a teardown and inspection, especially in a low compression situation, there can be many causes. Pulling the heads, not the engine, is your first and best course of action to determine the cause of this issue. If it is a valvetrain problem, it can be repaired with the engine in-place. As long as we're guessing at this point, that's my guess. This is just my two cents and I expect some change back...