is there a specific reason that you're using the late model engine as opposed to a JDM P10 SR20DE? you're kinda working backwards here... not that it can't be done, you're just going to run into wiring problems trying to add the missing OBDII parts that the newer engine and stock ECU will be looking for.
Reason being is that I have access to a '00 G20 motor (which now has a det), and the 91 se-r burns oil. The motor is complete with uncut harness and ecu. I would think that all I have to do is transfer everything, and in theory should work fine. Is there anything else that might be an issue?
I don't think you'd have any issues, but getting an RR SR is not the smartest idea. Just order a motor through Soko or JGY. IMO, Swapping in a JDM/UKDM motor is going to be way easier than doing the RR swap.