You need a Greddy Speed Cut Controller (SCC) It's just a couple of wires that splice into the ECU harness and remove the speed limiter there.
Call MVP Motorsports, they can set you up.
As for the ECU's, The US/Export Spec ECU's are set up to run a mass air sensor and different injectors, so there are complications for a direct swap into a JDM Supra.
Depending on your power goals, the stock JDM ECU can support a surprising level of power with the proper supporting components and tuning.