Carbon fibre (fiber) has a long way to go before it can stake the claims to simplicity, machinability, and relatively low cost that aluminium and (thin wall) cast iron have.
CF is expensive, cannot be machined, loses structural integrity if any of the fibres (fibers) are interfered with, and doesn't have the high heat resistance of metals.
What happens if you blow a cooling system hose with a CF block (a common occurrence), and the engine overheats? It will probably self-destruct with the unanticipated high heat levels.
I know CF is a popular product in the aviation industry, but engine blocks are an entirely different kettle of fish to airframe components and aircraft panels.
Why Is Carbon Fiber So Expensive?