Oh! Dear! Roger, you’ve raised an unwelcome spectre from my forgotten past!
Long, long ago I got my hands on a very, very ancient Johnson opposed twin outboard motor, it too had one piece cylinders+heads. While attempting to salvage the rest of it I decided the safest place for the pistons was in the cylinders. At some point I moved one of the cylinders and when I put it down there was an audible “click” . . .
Yup! You’ve guessed it. The aluminium cylinders had iron liners and I had jarred it enough for the piston to move and the top ring to drop into the “undercut” formed by the head end of the liner. The ring was still enough in its groove, (Sodds law), to prevent removal of the piston and the only point of access was through the spark plug hole.
Several weeks of off and on increasingly desparate fiddling later I did manage to compress the ring back into its groove and extract the piston.
I found another “safe place” to store the pistons after that!
Sadly the engine was never a total success and today it lurks at the back of my shed as another of those projects that: “I’ll get around to sorting yet!”