Just spotted this and had a look at the code for the sync protection crack. Have to say.... very, very clever method! I hadn't thought of being able to re-sync the code with the counter again every time the trace de-crypt is called. And also re-syncing to handle the trace checksum code being modified too! 25 years on and I just learned something about ST hacking.
Just goes to prove the theory of hacking, there's always someone smarter who can think of that thing you didn't consider when making the protection.