Would you please provide a link for that statement? To the best of my knowledge, Stirlings don't replace Otto cycle engines in automotive applications because they take longer to start, run at much higher RPMs, are bulkier and aren't suited to the variable power that automobiles demand.
A power plant, however, seems like a made-to-order application for Stirling engines.
On the other hand, newer Hybrids are using Atkinson-cycle engines to get greater efficiency from a gasoline engine at the expense of power. They're more complex because the pistons are connected to the crankshaft via a series of levers instead of directly as in the Otto cycle. This allows the engine to run with a longer power stroke, hence the efficiency gain.
Current small Stirlings I can find run about 20% and are sold as "low polution" stationary power sources in California. This one is fairly compact. Notice that what you can see looks like an aero engine, and is sure to be very expensive. Pretty, though. Maybe 30 kw, about 45 - 50 horse power. Obviously hand made.
These machines are obviously not ready for prime time.