I prefer a chain wrench that can wrap around the cylinder head but whatever method you use don't worry about finding a seal. They are available in almost any size as are O rings which I doubt it has but. The piston seals are likely cast iron piston rings but can be replaced by floating seals too. They probably are not the culprit however.
If you think this one is tough be glad it isn't a cylinder with a weld head. Now those can be fun. Think lathe.
