How much are you willing to pay?
Stirling engines would be cheap to produce in mass quanities and really don't require that much material. Every new technology is expensive, but once it is accepted and mass produced it becomes relatively inexpensive.
My response was to all the negative comments. You can have a stronger lightweight hybrid car using new (or soon to be invented) composites. These cars would could zip along rather nicely, charge superfast using nano-technology batteries, and with solar powered Stirling engines not need to be plugged in for trips to work, mall, and food shopping.
The future is coming whether we like it or not.