Find a
FIRE engine for full on MPG and modern technology / MPG. As rmay pointed out the dimensions between engines remained the same so mounts can probably be bought or fabricated to suit.
Or better still how about the FIAT 128 engine spec
146.A8.000 from the Uno Turbo 1.4 IE - turn up the turbo and thrash anyone with a 350 all day long, better MPG too.
There is a replacement for displacement, its called a Turbo