Folks,
To be fair to CASA (and I am not a noted CASA booster) all they have done is enforced compliance with the Cessna MM.
FAA don't have to do anything, the Cessna MM automatically applies to any Part 135 or up use of a C441, so the result is automatic. For a privately operated N registered aeroplane, the Cessna MM is not automatically mandatory (but do check with your insurer).
There is always the option of a major repair scheme, which would probably include rebuilding the whole pressure vessel. Economics will dictate that one.
Tootle pip!!