Actually Festiva's are rebadged Mazda 121's or Kia Pride.. Kia Pride actually since they were first. You can put several motors in them from the 1.6L (SOHC or DOHC) in most mazda's clear up the 1.8L (SOHC or DOHC). Common swaps are the turbo models from the Capri's and the BP variety from the Escort. They still get 35+ but if your looking towards mileage I wouldn't.
Getting a stripper model and adding the Aspire head on the Festiva block would give you the most efficient engine setup. Aspire's use rollers.. less friction. The Aspire trans isn't geared as high as the Festiva's. Don't know if it would help any in the mileage though.