I looked up the 1984 on
www.fueleconomy.gov/, and the diesel version was rated at 21 mpg (combined). It might be tough to find one, but I would start there. Before jumping into an engine swap project, drive it like we know how and do the standard mods; grille block, tire pressure, wheel discs, belly pan, etc. I'd think 30 would be achievable. Plus, you could run veg and smell like french fries. Bonus!
Or, if you're bent on the swap, buy a Geo Metro for your daily driver and make the El Comino an EV project! Now that would be cool.
D'oh! Daox beat me to it. ha ha!