@itsgumby, I know about the RPM signal and the thermometer should be simple too (apart from the calibration stuff). The speedometer wire in carby cars should also be straight forward, and I dont use CNG so no idea about that. I installed a micro controller based RPM meter in my previous Cuore, I think I should be able to find its circuit diagram, but the idea is simple that you use the rotating point in the distributor and tell the system how many cylinders and what stroke engine you are using and put all the variables in the formula to get RPM figure.
For temperature you use a kind of chip which changes resistance according to temperature so you need to find one with the range of the engine and calibrate for certain values what was the actual temperature, but maybe Asif had the engine's temperature wire, I do not know exactly how it works but google maybe able to tell you what technologies are out there and then you can test which one your engine uses.
I am rather interested in replicating scangauge.com so a screen and an EOBD serial chip coupled with an Atmel 8052 should do the trick and bring down the cost and eliminate import duty 
