Go to Napa (or the like) and get yourself a spray can of gasket remover. Spray liberally on the affected area, and wait about 20 min. Then srape off the goo with a paint stir stick (wood is best, the gasket remover will melt some plastics) DO NOT USE A METAL SCRAPER! You will gouge the sealing surface and then... no more seal...
You may have to do this 2-10 times.
When its all off, (and make sure it is all off) be sure to clean all of the gasket remover off the head. It will eat your new gasket too.
As for the pistons, some people would say to just leave it there.... I'd take it off. Excesive carbon deposits lead to premature combustion. The carbon heats up red hot and ignites the fuel mixture in the combustion chamber before the spark plug would. Your ignition timing may be correct but the pistons would be firing at the wrong time anyway.
You can soak the pistons in Varsol (mineral spirits from auto parts store) and use wood or plastic to srape off. You don't want to nic the piston crown with anything metal or you could get the premature combustion thing happening all over again. Take the pistons off because you don't want that carbon and Varsol in the block. Clean and Smooth is what your aiming for on both counts
Hope this helps.
