Another problem with "crate engines" is that the ECUs require inputs from multiple sensors or devices. The days of simple cars died with the Rover V8, I feel.
As cars get more like computers this need gets ever more difficult to manage: Simon mentioned that the Jaguar V8 needs inputs from many, many things, including the steering pump, I guess because as the load increases the engine has to respond. If it isn't a Jaguar steering pump it won't play.
This need for support may be why MMC seems to be getting further and further into bed with BMW.
