Analog/universal input HUD?

I am getting ready to do my first TSD and wanted a more accurate speedometer, so I downloaded an app (DigiHUD) which uses a GPS speedo and projects it mirrored so you can use it as a HUD speedometer, and this got me interested in actually installing a HUD in my RX7.

The problem is that all the aftermarket HUDs I can find are OBDII... which my 40-year-old car is not. I started searching for individual components, namely a tach and a speedo, and I was able to find a GPS speedometer but not a universal tach that would be compact enough to project onto my windshield.

Is there anything out there that could take inputs and convert them to OBDII? I wouldn't have an issue changing my sensors or adding new ones (say if this box uses GM temp/pressure sensors) but it would need to have a coil- tach input and preferably a GPS speedo.

There's nothing available off the shelf, you'd have to make something yourself, perhaps by programming an Arduino.

There used to be non-OBD HUD units available that would take generic PWM/pulse inputs, can't find any now though...

I looked into this a year ago. Raspberry pi, and there’s a program, I can’t remember what it’s called. But most of the sensors you have to plumb in from other obd2 cars. They don’t have to report to the cars Ecu, just the Pi. Which you also have to build. 

https://m.youtube.com/watch?v=JaF25oIVG08

edit: https://youtu.be/5C9ypE6JUuY