Engine ECU sends engine speed, torque and TPS directly to the gearbox ECU and that in turn sends a timing retard signal back to the engine ECU to give smooth gearchanges. Then there are the other bits that need to know the engine speed (or that it is even running), a signal from the HEVAC tells the engine when the AC compressor clutch is engaged so the revs stay steady too, even the heated windscreen will only switch on if the engine is running. There's so much interaction between the different bits that fitting the engine is the easy bit, making everything else work is where you run into problems. That's why I say fitting a Motronic controlled L322 engine into a car that had a Motronic controlled 4.6 engine could be possible. Fitting a modern engine, such as a Lexus unit, into something very basic, like a Capri, is dead easy, you just need to use the engine ECU that came with the Lexus engine engine to deal with ignition and fuel injection. The car isn't fussed what is making it move as long as something is.
However, as OB says, as long as the original engine is looked after, then there's no need to think about changing it. I suspect most of the projects that have been started have been because someone has blown up their engine and have something laying around gathering dust so they decide to try to fit that rather than spend the money repairing the one that will work.
