Engine Material Composition?

Can you provide a little more information? What type of engine? Who made it? What model? What duty? What is it driving? Is this an engine you own or are you asking a theoretical question?

Based on the information provided there's very little that anyone discern about it.

Pistons are generally made of some Aluminium Alloy as are modern engine blocks. Blocks years ago were cast Iron for rigidity, but were heavy...

I have never heard of stainless steel being used personally, probably too heavy and not got good characteristics in the strength department either....

Yes right. Deltalloy®4032 (Alcoa's brand name) made it one of materials for pistons of diesel and gasoline engines, the reasons being:low weight, high thermal conductivity, easy machining, reliability, higher fatigue resistance in comparing with cast iron, and perhaps recyclability with optimized conditions in respect to chemical composition, production and heat treatment.

Compound ceramic has been made use as well for piston rings.

Forged aluminum. Not the usual 6061-T6. Part of the reason is thermo expansion. The piston can't glow and shrink too much.

Pineapple