91+ is DOHC. 90-91 a lot changed. Try the features and specs section on
http://www.socal240sx.org for a list of features on diff model year cars.
As for exhaust...I have a crush-bent 3" catback with a PDM-Racing Border muffler, carsound cat, and ebay downpipe on my CA, and it sounds very deep. My friend's Apexi catback sounds mean on his Sr, too. As long as you avoid the APC-ish garbage exhausts, you should be ok.