I don't think you will get the cylinder and head off in one piece on the 12+ chassis. She's a pretty tight fit which takes some holding of the tongue in just the right place even with the head removed.
If the O-rings only have 20hrs on them I'd inspect and reuse. If not for $10 I'd replace them.
No point wasting money on rings/piston at 20hrs. By all means measure your end gap and compare to what they were installed at, but I'd say they'll still be well in spec.
Any comments/feedback on what you dislike about the current setup driving the change?