Funny you should ask that!
I *just* bought a kit air/fuel mixture meter.... From my place of work. "Silicon chips" magazine produces it. It cost me 55 aussie dollars. Cheap enough! It doesn't do CO though, but does the idle air mixture, so i assume it's a similar thing. It doens't need to heat the sensor, all it does is tap into the existing lines for the Oxygen sensor. You say your's doesn't have an oxygen sensor? I thought it had to being a 1990 model, anyways, just go to a wrecker, pick up a bosch sensor that will fit the screw thread in front of your catalytic convertor, then you just calibrate your own meter (after you use a +12 v supply to heat it!) anyways, Ciao, might start the kit!
Az