1993 Lexus SC 400 Engine Replacement

I got this car used a couple of years ago with pretty good mileage for a 1993. But its causing me some problems right now. Here goes:

About 5 weeks ago, I had the timing belt break. But I was driving at low rpm at the time. Got it towed to my mechanic and he said replacing the belt may work or it may have caused major engine damage. I elected to replace it with new bearings and get a new water pump($1700). Got it back and it ran fine for 2 weeks then the engine randomly died on me again. Got it towed to the mechanic and he said the lower end of the engine is damaged, and the cheapest option is too put a low mileage used engine in and the quote he gave me ($2500). After much thought, I decided to go ahead with it. And I said that would be all the money Im willing to put into this car.

NOW.I go out of town and come back 2 weeks later expecting to get my car back and he said they replaced the engine twice because the first one wasnt working that well. Then he tells me. The second engine cranked up and ran for 2 hours. Then when they drained the fluids and put new fluid in. It cranks but the engine stalls. But couldn't give me a reason why (All he said was he thinks its an electrical issue). Now. Some 2 weeks AFTER that. He finally narrows it down and says the ECM is fried and there is no output signal going to the fuel injector. He says its because a few electric wires were corroded.

I am unconvinced that the ECM failure is un related to them replacing the engine (All you need to fry a computer is a surge of voltage which can be caused by a defect in a sensor, or putting the wire in the wrong slot when the engine was being replaced). So I told him I am only paying the $2500 I originally said to get the engine replaced.

Am I justified in my approach. Or is the ECM failure absolutely 100% without a doubt not related to the engine being replaced (because If someone can convince me of that. Then I will pay for the new ECM)