Not sure if this were ever answered?
XXO is pre-1956 pushrod L6 etc. using some head other than the original casting. Since fabricated is legal, it doesn't have to be of pre-1956 design and manufacture.
Does any head qualify as legal as long as the pushrods & rocker arms are operated by a cam in the original position?
No, I won't live long enough (or have enough money) to attempt this, but it appears to me that there are modern OHC engines where the head casting has enough features in common with an XO engine that a rocker assembly can be adapted and run off (new lobe positions) in the original cam & drive.
Specifically: Chevy 235/261 with Ford "Essex" 3.8 60° V6.
I've made some notes on this for anyone interested.