Pretty much of what could be doable at a reasonable cost to improve the efficiency of a reciprocating engine had already been put into commercial production, while a handful of other configurations such as opposing-piston are slowly making a comeback, but most of the internal components are not going to have their basic designs changed to such a greater extent. OTOH there's still a lot of room for improvements at the engine-driven accessories in order to decrease the load they add to the engine.
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