If you're in a hurry, you can get groupheads from espressoparts (you may have to call them, or dig through their poorly organized parts collection) in order to get the thing you're looking for. It'll be in great condition, shiny and new, but it'll be pricey -- like $400-800, give or take. You might do as well to find a "cheap" knock off from a similar high-end espresso machine. Watch e-bay for commercial espresso machines that are broken and being sold "as is" for a much better price. If you're seriously interested, it would also be of use to start hanging out at coffeegeek.com, reading the alt.coffee forum (google groups has alt.coffee available) and reading Schomer's work at lucidcafe.com. Schomer knows how to make coffee.
But then, I was mostly joking. The engine block espresso machine has some excellent selling points, but also some huge weaknesses, in all seriousness. Ensuring uniform block heat is liable to be a real hassle. If you can manage to heat it uniformly, you'll be likely to burn yourself on 95C metal at every turn. There will definitely have to be some custom work done to make it work like an espresso machine (think La Pavoni lever-operated machine) rather than a sports car engine: you'll need something different for rings to seal the pistons into the head, and the valves probably won't hold that much pressure for that long without some kind of modification. It'd be outstanding to have a beautifully chromed and polished engine block putting out coffee, and the thermal consistency would likely be fantastic, but getting there from here is something of a challenge.
If you could manage this, you'd be my complete hero. If you're not a fairly serious espresso and engine geek, however, it's probably over your head.
Dan