Jim Stewart wrote:

Based on what I've seen, it's hard to imagine
a Stirling that could fit into an automobile
and produce meaningful amounts of power.

The Philips company has made Stirling cycle engines to run city busses.
Getting that kind of power out has many complications, and usually uses
helium at very high pressures as the working gas. Non trivial sealing
issues as a result.

Making a Stirling that runs is trivial, comapratively, to getting one
to run at or near the peak of efficiency.

Cheers
Trevor Jones