1990 Lexus ES 1990 Lexus ES250 knock sensor replacement

The Check Engine Light is on, code indicates a knock sensor failure of some sort. Engine currently fails [MD] state emissions tests for high NO2 (needs EGR valve?), But otherwise runs fine. The knock sensor lists for around $200 (I may need two?).

Does anyone know where are the darn things are physically located? (My crappy repair manual just says atop the engine block.) What does it take to change them? Is it worth it?

I'd especially be interested in feedback from someone who has actually done this themselves, or knows what's really involved, results, (cost?), Etc.

Thanks in advance.