Not sure what you mean by re-flash. Re-upload stock firmware or modify the firmware because I think they use the same term when they are modifying the ECU (correct me if i'm wrong).
Why do you want to re-flash the ECU? If you're in California, I think they were banning the modification of the ECU. When you go for smog check they would know if the ECU was factory or if it has been modified. Not sure where the ECU is located on the 1st gen but I think it would be easier to get a second one and plug it in when you do smog check.
If you want to reflash the ECU go to the places that do dyno-tuning.