I remember seeing a few engines with an eccentric bushing for the crankshaft so they could change the compression of the engine. It should work pretty good as it would eliminate the need for a variable compression head with its potential problems with the contra piston.

But for four stroke engines the eccentric bushing is a problem because of the camshaft gear setup, unless it used the rear camshaft arrangement. But maybe a belt drive cam setup would work Ok for this too though.