There are people around who specialise in auto electronics. If you can't find a firm who do car electronics, then a television or audio repair company would be the next best thing. Most ECUs are less complex than the average PC, and in most cases are damaged by an event that occurs outside them, in the car wiring or sensors.

The most common cause for failure I have found when repairing ECUs is with the electrolytic capacitors used to filter the power supplies. These are small components filled with a liquid or jelly. In Japanese ECUs the fluid commonly leaks out, causing problems to the rest of the circuits as well. In very old British ECUs they dry out, and fail that way. The components themselves are very cheap, and any electronics technician will be able to change any faulty ones.

Can you find out from the firm that repaired the ECU initially, what they exactly did? That would be useful for anyone looking at it subsequently.

Go back to your original repairer and get them to have a look at it. If it is still repairable they may fix it at no further cost.