The V8 engine that you buy should come with it's own V8 ECU. This will plug into the existing 2002 WRX harness.
Later, you can add the AVCS harness, the easiest way is by buying it from IAP.
Looking at the diagrams, some of the pins seem flopped for the JDM ons2.
But I am getting reports of form members being able to use obd2 scanners with the JDM ECU.
Edit: I revisited this issue with greater detail, OBDII scanners use ISO Protocols. These can differ in the pin that is needed for communication. SOme handheld scanners are multi-prtocol and can scan several pins, until they actually connect.
SOme otherones look just for the pin that has a different connection for the JDM.
Subaru uses a different protocol, the flashing and reflashing software all work, I think due to this: Ecutek, Ecuedit, Ecuflash, Romraider logger, Ecutek data monitor, Ecutek delta dash and road dyno, all work, with no repinning needed.
However, a state inspector computer could pull 3 different kinds of info that have to match: year, country and model.
So even if the state inspector computer can now communicate with the JDM ECU, they will expect to see: 2002, USA, WRX and they will see 2004 Japan Sti.
So getting the JDM ECU to do OBD2 is.... not that desirable...