Allessandro, The piston tops need not be sparkling. Simply scraping off the loose carbon (w/plastic tool) and blow it away. If you don't have access to compressed air a can of computer "compressed gas" should suffice. I put a balled up wad of paper towel into the coolant passageways and duct tape over the oil passageways when scraping or blowing. The machined holes for the headbolts get blown out of oil and debris with air (hold a rog over the opening so you do not wind up with a face full of @#$%%). You can rotate the crankshaft to bring each set of pistons to the top. The pistons do not remember whether they were last on a compression stroke or exhaust stroke. Just set the timing marks as needed when installing the head and timing belt.
The B21-B230 are very easy on cam and valve seat wear - unlike the B18-B20 motors. Unless they've been run dry of oil they rarely need re-adjustment.
A pulled back photo showing all the cylinders would be good to have. - Dave