For peace of mind I've poored oil inside cylinder to see if there is any leak but it's fine.
I did another leak test on valves, this time removing the exhaust pipe, and they are fine, no leaks.
Calamine should cover those pings in a few hundred kilometers, making the whole piston head back to an almost normal surface. Calamine on injectors makes them almost impossible to remove so I guess it is good hard stuff.
My cylinder wall is intact I had chance the torx screw head crashed almost directly in the piston and stucked there after 2 or 3 bumps, preventing it from doing more damages.
The part was light and tiny, and made of a tender metal that breaks easily, so I don't think it could have caused damage to valves or what's under them. Same with rod. Even with the force of explosion. It was enough to make a few dents on the aluminium but valves or rods are made of a harder material so I guess I should have luck. I don't have the tools to check this, and mechanics in France don't work for a drink unfortunately, they have work above their head...