I'm fairly certain that removing the head is not necessary for rolling in a new timing chain...However, it might be less if you have to do the tension, guides, or rails. I'd measure the chain stretch before making a decision (there is a wiki with the method:
http://www.peachparts.com/Wikka/OM617TimingChainStretch). There are also plenty of pictures so you can get a feeling for what's what.
What is the mileage on the vehicle? Has it been maintained regularly in the past?
PS. You'll get better responses if you post in Diesel Discussion!