If you did get a JDM GTE and it came with the ECU and wiring harness, then theoretically you should be able to just drop it in (barring pan differences) and be all set right?? You should be able to use all the accessories that came with the JDM once you rebuild the motor. I might be wrong, if so someone please correct me because I am going to buy one very soon for a GE to GTE swap.

You may need to swap some emissions stuff over (depending on your local laws and if you choose to follow them). As for the harness, I'm pretty sure that on the JDM Supras the ECU is going to be on the left side of the car, not the right like the USDM Supras. So you would want a US harness, some of the sensors, and then your ECU and engine should work fine. Or you could lengthen your JDM wiring harness.