As you have discovered the HCH1 fan is intelligent and can't be powered up by the simple application of a 12v supply. It's a hi power fan and needs a 12v higher current supply and
a pwm signal to control the speed etc. Jeff652? on here sells a special gizmo to power the HCH1 fan with a grid charger.
Now you are asking about the ECU, but do you really mean the MCM which is the silver 8" square box in the IPU compartment which sends control signals to the fan or the ECM which is at the front of the car behind the glove box.
I suspect you mean the MCM. No one has a full schematic for that, but the workshop manual does contain some details and may show a transistor on the relevant output from the MCM. I don't have access to mine at the moment others on here may be able to oblige with a scan/pic.
You say you now need a new computer, did it let out smoke or do you now have an error code etc, what code do you have?
What makes you think it is damaged. They are very tough with well protected I/O.
The parts in the MCM are tiny surface mount jobies. You may if you are very lucky be able to trace the MCM pwm fan control wires back on the MCM pcb for a short distance, and if the gods are smiling on you, you may find the relevant transistor. It's likely a simple npn type, so something like a 2N3904 or BC337 may work if you can't identify it's exact type from any markings it may have.
The difference in the variations for the MCM programming are quite subtle but basically any MCM from a manual car of the same model 2003-2005 should work. Don't get hung up on the exact version number.
If you take the top off the mcm and find any damage or identify the relevant transistor please post some good quality pictures so we can advise further and others can also benefit.
Please update the thread with the outcome either way and update your profile with your location, this is an international site someone may live near you who can help.