[Big Dave]

Another possibility would be the Mercedes-Benz I-5 turbodiesel out of a Sprinter van. These are just now becoming available at recycling yards. Sprinters get fabulous mileage but as a van they are slow, slow, slow. Getting the DIN bellhousing to match up to your Japanese transmission might be a machinist's nightmare. You'll need to regear the rear axle. Details, details.

An engine that gets nearly 30 MPG out of a van should give you staggering MPG in a Z.