I have a 1990 S13 with a KA24DE. My IACV is broken but uses the old square connectors. The harness side is an 8 prong connector, but has 2 of the prongs missing (looks like it came that way). The new IACV design has round connectors has 8 prongs (none missing) on the harness side. Does anyone know if I can swap the sub-harness and use the newer IACV design? Or are those 2 extra prongs going to be a problem?

https://www.wiringspecialties.com/ka24de2.html

To replace your broken connector. To answer your question, I'm not sure about swapping IACV style. I assume the stepper valves work the same but... I honestly don't know.

One option is to find a used IACV like yours and clean it really well, then repair your harness connectors with those WS units.

https://ratsun.net/topic/38676-ka24de-s13-vs-s14-iacv/

S13 vs S14 IACV discussion https://ratsun.net/topic/38676-ka24de-s13-vs-s14-iacv/

Not sure how accurate that is. somewhere to start at least.