Or a 1st gen Insight? That would kick butt.
For those who don't know, the 1st generation Smart Fortwo diesel had these specs:
engine: 799 cc common rail diesel
power: 30 Kw / 40 hp
transmission: optional 6-speed electronically shifted
weight: 730 kg / 1609 lbs
fuel economy: average 4.2 L / 100km (67 mpg Imp. / 56 mpg US)
The only CDI drivetrain swap I know of went into the aero/tandem Jetcar:
MetroMPG miscellany - MetroMPG.com
I wonder if you posted at clubsmartcar.com whether they'd have any more examples. Though my limited time spent there suggests they're not very excited about efficiency mods.