Thanks Kaz for your thoughts.
I fitted the HKS unit in the loom before the ECU using the Japanese instructions and a Japanese NSX friend who translated, unfortunately it still is restricted. So that didnt work! I was going to temporarily swap ECUs from the Australian car into the JDM car to see if that worked on speed limit and cruise limiter, but the JDM car has 4 O2 sensors and the Aud car has 2 O2 sensors so thought that wasnt so smart.

Does anyone know which ECU from a USA car would work in the JDM car? I guess a DBW with 4 O2 sensors but think the USA CARS are OBD2 and the JDM is OBD1 for that vintage.

Thanks,
Scott