The Esslinger engine is built on the specs for the 2.3l OHC motor and it uses several stock (or equivalent replacement) parts.

You can bolt their heads to a stock block or vice-versa...they use OE headgasket, rear main seal, valve cover gasket, main bearings and rocker arms, along with a few 2.0l EAO-based parts in the head (adj. studs and springs).

The valvetrain on the short heads have stock geometry...the tall heads have custom stuff, but the short heads are just worked-over (better ports) aluminum 2.3 Ford head.

The EST has even more stock parts in the EFI system.