Heathkit? Oh, my. That company made kits that hobbyists built (radios, TVs, etc.) until 1992. So if you are looking to buy a Heathkit exhaust gas analyzer, you are getting something MADE by a hobbyist, probably at least 30-50 years old. I would not think that the sensors are still that good, and I would also think sensor technology changed over the ensuing decades.
I think it may be better to find someone with a more modern unit and go from there.