As far as Spitfire seat alternatives.
Did the Miata seat conversion twice, didn't like it either time, took them back out. Besides seating you absurdly high, they cant in, aren't centered, and have little travel room.
Saab Sonnet seats do fit. I have a set. I wouldn't consider them an improvement over the stock seats. Starting with their lack of recline adjustment and their skinny deep bucket for your posterior.
Fiero seats do not fit into the car. Tried it, they won't fit into the space between the driveshaft tunnel and the sill.
I have heard that some 4x4 seats may fit. No experience, never heard anything more from the fella who told me he was converting to one of them.