Neil, I still drive this Subaru Forester 1999, it has an EJ20 engine. The factory cylinder head gaskets are 1.6 mm thick, I replaced them with metal ones of 0.6 mm and installed the heads from the EJ18 engine, which have a smaller combustion chamber, the math shows a CR of 1:13. Of course, I adjusted the tensioners of the toothed belt, so that both heads (box motor) are in phase alignment and the opening of the valves happens a few degrees later.
There is something in the connection that allows any engine to work well regardless of CR, that is a well-prepared mixture of fuel and air and, of course, a quality spark plug, such as NKG LPG1