Well you couldn't simply "modify" it, since the 300HP version has 10:1 compression as opposed to 9:1 for all US motors. So you would have to have a euro motor actually built...pretty much from the ground up. Check out www.mercedesengines.net.
They supposedly don't do it, but I have heard many times that if you shell out a few extra hundred dollars they will indeed build you a euro engine. Go to the site and you can also get an idea of price. Also...do a search on this topic, many have written on it before.