It isn't exactly "Plug and play" but I know the GM Ecotec line has been swapped in. They were often turboed stock. Low displacement, and hopefully decent MPG
Others have swapped in the Quad 4 motor. I am aware they are often built to be really high horsepower. I don't know where the mileage numbers would come in. I've also read it is not an "easy" swap either.
Depends what you mean by plug and play.
1985 Pontiac Fiero 2M4