>>1619509This research uses the micronutrients and minerals from agricultural waste in aluminum alloys:
https://www.intechopen.com/books/the-future-of-internal-combustion-engines/a-new-lightweight-material-for-possible-engine-parts-manufactureMaybe carbonizing the waste before using it would improve the results.
And there are engine layouts, such as VR engines, which are compact and propagate stress in different ways, for example piston ignition acts diagonally and to the outside of a single small block in the case of VR, with a small "dead space" in the diagonal of each cylinder, and this can faciliate the use of composites. For example a strong outer skin holding the block together or some metal foam in the dead spaces.
https://silodrome.com/horex-vr6/ 