Are you positive the ECU (technically its the Powertrain Control Module PCM) is damaged?
The PCM has an immobilizer feature to prevent car theft, and if it interacts with modules that do NOT identify themselves properly, will result in the PCM shutting itself down and refusing to start.
If you've restored the vehicle to its original state, and it still won't start, then you may be right, they fried the PCM/ECU.
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