For the LCD gauges have you considered getting a harness out of a newer (OBDII) car and making it fit so that you can simply use the software that comes with the readers available? I would think that someone over at Miata.net or maybe even the guys at Flyin' Miata would be able to help with that, and judging by the work already done to your car I think it is easily within your skill set. Just a thought.
---------- Post added at 10:11 AM ---------- Previous post was at 10:03 AM ----------
...one more thought - though I don't know how much you want to go overboard with this.
A GEN III small block Chevrolet would get you that OBDII.