Check some of my videos. I've shown piston removal. Learn from my mistakes and don't do it like I did it. I cracked a cylinder.
You will need to grind a socket down. If the motor is stuck, you might need to also grind various wrenches to get onto those nuts.
How not to remove pistons.
https://youtu.be/SlYZGlONTYs
How to remove a crank from a stuck motor:
https://youtu.be/b-dPIVcy1sM
Re the pistons. My mistake was to try and remove one that still had the full circumference of the crown in place. All the others had at least a part of the circumference missing and came out ok.
Mart.