1974 Brain Box saves the day.

If you can get one apart and clean, it should be easy to duplicate the circuit, but getting original value components might be tough, as the transistors and such may be (sometimes purposely) unmarked.

The real "gotcha" on more modern electronics is multi-layer PC boards which can be a ***** to trace. I am not good at reverse engineering. Frankly it's not worth my trouble. I'm happy to use GM HEI modules--so long as decent replacement ones are still available

The other little advantage to a GM unit is that with the key in "run" and engine stopped, a proper GM module will not draw coil current