Speedo Problems!

I can't see it being easy or cost effective to convert an electrical signal to a mechanical speedo. I would suggest trying to mate a speedo out of a newer celi or mr2 onto/into your cluster, but I have no experience in this area.....just an opinion

 

Yeah i think schmooot has got the right idea, you'll need to remove the original mechanical speedo out of your cluster and replace it with one out of an MR2 or similarly compatible model to the gearbox.
I'm not entirely sure on the workings of an electronic speedo though providing it doesn't need to pass through the ECU, which i don't see why it would need to, i think it'll be a medium to hard job to do, thinking very basically just remove the mechanical speedo lines, replace with the electrical speedo cable and find a power source, still not knowing the intricate workings i can't be 100% sure. The hardest part i think will be getting a hold of the compatible speedo and cable, might be best to get onto some MR2 enthusiast sites and see if anyone there can help you out with details on the speedo system to match that gearbox.
Just my 2 cents.

EDIT*
Just did a quick read up on Speedometers and how they work, not entirely thorugh it yet but there is a mentioning of a signal needing to pass through a computer so i could be wrong about the wiring not needing to run through the ECU.
check it out for yourself here.
http://auto.howstuffworks.com/speedometer1.htm

 

All the newer cars I've seen it runs through a module....I'm usually looking for a speed sensor in my line of work and its usually a very fast negative pulse that I either pull off the cruise control module harness or I have to go to the PCM or BCM