There are some really beautiful examples of an mx5 that has been amended to a fastback but these appear to be mostly american. I'm liking the mx250 but that's a whole lot of work and modification for the normal everyday person.
Have you considered a possible "smaller" kit which would convert the mx5 to a fast back? Essentially a roof and boot mod, perhaps even covering the arches as well.
In theory, the same length and width being kept on the car, just updated a couple of bits (fast back roof, new boot lid and a flush for the remainder of the boot which inevitably would not be covered completely.
Eg:
http://www.mazda-speed.com/forum2/in...?topic=22020.0