While engines are critical, and expensive, it isn't the only part of the aircraft that the ash damages. If I recall the KL incident correctly ash got into just about every nook and cranny of the aircraft. The basically had to take the airplane apart and put it back together again. It took almost a year and $80 million to repair (I think that included replacing the engines).
The only reason why KL (and their insurers) didn't scrap it was the long waiting period, at that time, for a B747-400. The official report said:
The windshields were damaged, as were internal aircraft systems, avionics and electronics.
but that's a bit of an understatement.
Anyone from KL who can give us more detail (and correct my memory, if appropriate)?