Here is a Heathkit. Seems pricy though.

https://www.ebay.com/itm/VINTAGE-HEATHKIT-MODEL-CI-1080-EXHAUST-GAS-ANALYZER-1974-/132921577917?_trksid=p2349526.m4383.l4275.c4#viTabs_0

If you don't find one, drop me a PM and I will dig mine out. I am well retired and no longer work on air-cooled VWs.