Saab has a proto-type of such an engine.
The displacement varies by change cylinder head volume.
Basically the head pivots in relation to the block. When its up-- its at low Comp. Pivots down and is flat to the block like conventional head (high Comp.)

I've yet to fully understand how the head seals and doesn't leak if I did I'd likely be working at saab

Anyway-GM-is exploring this option which means you have hope they will screw it up.

My current engine dreams are associated with a fuel injected weed wacker.........I hate smelling like 2cycle........