Plastics don't have the strength required for blocks and heads, and can't withstand high temperatures. Intakes bear no structural loads, run relatively cool, and are ideal for plastics.

Plastics "creep" under load and temperature, so are not very stable in many applications.

Expect more hybrid applications of plastics - like plastic brake reservoirs on alloy master cylinders, or plastic tanks on aluminum radiators we have now.

Maybe plastic alternator housings, water pump housings, thermostat housings, timing covers, valve covers.

 

Ford has a 3V head, but its SOHC, not pushrod. The big deal with this is that its pushrod actuated, not overhead cam.