It looks like a handful of people have made the Miata a good conversion vehicle. Here are a few from evalbum.com. They all went with a fairly high voltage even, so you know they're packing a buncha batteries in there.
Pat Mackey's 1992 Mazda Miata
Pat Beirne's 2000 Mazda Miata
Dimitri Butvinik's 2001 Mazda Miata EV