is there a way to polish or clean the piston head without changing too much of its performance or maybe wearing it out? I know I cant see into the engine, but I opened up my OS91FX recently to have a look, cause I saw my plug was being burnt black which generally means im running way too rich... so now the piston is also burnt black on top... I want to remove it if possible or doesn't it really matter? I have tuned leaner now... I was about 4turns out... im at 3now... manual suggests 1.5 to 2 turns but thats too lean... the engine died at top end so I richened until this didn't happen anymore and I am at 3turns currently...

This is overdoing it, do not fix problems that do not exist, especially if you are a beginner.

John

If it's just discoloration, then there's no real need to do anything...that's normal.
If there's substantial build-up (carbon, varnishy gunk, combination of both ) and you already have the head off, you might as well clean it. "Substantial" build-up meaning perhaps a mm or so thick.

I typically use a whittled down popsicle stick to scrape as much as possable off...the stubborn bits can be carefully removed with a relatively dull edged metal scraper (w/ rounded corners ), being careful not to gouge the piston. Scotchbrite scrubbing pad for polishing.
Put the piston at Top Dead Center...then after cleaning, rotate the crank just a little to drop the piston so you can wipe away the crud from the upper cyl. without getting any down into the engine.
Q-tips come in handy for clean up.

You might consider doing a search on the forum for engine tuning tips too. You need to be a touch rich off of peak for good engine life, then spend the rest of your tuning time tweaking the low end for proper idle and transition.