Some pictures :
At first I thought the compression loss was from a leak of the cylinder head gasket. I changed it with no difference.
Then I took a look at the cylinder and piston...
Totally fucked up. the bottom piston ring is even set!
The crushed brass filter I found inside the crankcase
which was originally inside the screw..
The cylinder/piston is no more usable, here's a new cylinder! Not brand new but in pretty good shape.
I have cleaned the crankcase with gasoline and turned the crank to help metallic particles to fall off.