Human bones are strong, but not as strong as steel(unfortunately). At least not by volume. They may be stronger by weight though. Human bones are strong not so much because of their structure, but because of the materials they are made of. There are materials stronger than human bone or steel by weight. However, the material has to have other properties in addition to strength. It has to be tolerant of high temperatures and pressures and not too brittle. Just as importantly, it can't be too expensive. Steel is extremely cheap.

Birds have very light bones because they are hollow. Engine blocks aren't exactly solid metal either. They have hollow cylinders and water channels and oil passages.

It is possible to make lightweight metal foams. However, like all foamed materials, it would tend to act as an insulator, which might cause the engine to overheat. Still, this might have potential.

I know you are a fan of electromagnetic motors, which would avoid the heat problem. However, a piston engine style electric motor will always be heavier than a conventional electric motor due to the increased part count and needlessly spread layout. Regular electric motors concentrate the same size electromagnets in a smaller space.

There have been experiments with ceramic engine blocks and I think there might be an efficiency advantage in a hotter running engine although it might be possible to incorporate metal fibers in ceramics to conduct heat. I am not sure about the heat resistance of carbon fibers which would definitely be lighter than steel and strong but I doubt that they are cheap enough yet to be economic.

Engine blocks are surprisingly hollow. Including the hole which the piston goes through,there are galleries for coolant, lubricant,and mechanical pieces.

All that really can be done is to make stronger metals you can use less of, or lighter metals or other materials just as strong. And that applies not only to the engine, but the car as a whole.

many engine blocks are already made of aluminum to reduce weight, for higher performance, some have steel sleeves in the cylinder, and there are even a few engines out there that have a steel sleeve, then a layer of high strength aluminum, and a magnesium body, which is very light, but not that strong( hence the layers) but extremely expensive to produce.