I have a Honda 100 and only recently its muffler has started to give black smoke. I discussed it with an Antenna Dealer mechanic, he asked me to let it be like this for the time being as the problem is not severe yet, and when it does become severe, he recommended me to have the ring-piston pair and the HEAD replaced then. He advised to replace head too because he said having it sleeved would cause it to become overheated.
Any metallurgy experts here can comment better, but I think head needs replacement because the material to be used in sleeving the head cylinder is different from the material used in the cylinder itself. This difference may be the cause of uncontrolled heating of the engine head.
So, did you have the head sleeved or replaced with a new one?