My favorite under-hood cleaner is a 50/50 mix of ATF and kerosene. Will not harm wiring looms, aluminum or painted components.
Spray mixture on, wait 5 minutes or more, brush or scrap (or tooth brush) heavily soiled areas then water hose off and follow up with a very light spray of ATF/Kero and rag wipe to finish.
Works for me; been using this method for over 60 years. And yes, to answer your question, ATF was available 60 years ago...
Hardy Handshakes, Swede4