Would not like to comment on de-carbonising the combustion chambers Don't normally do that to a diesel do you?
BUT the cooling system
You don't say if you have direct cooling and if so if you are on salt or fresh water.
Direct cooing with salt water is the worst obviously. Freshwater cooling should not cause any problems esp if indirect
The water distribution channels in the exhaust manifold that take water to the heads get choked up.
There is no forced flow through the cylinder block water jackets ... they get filled with scale and other crud.
Chemical cleaning of the blocks may be a possibility but the only real way to properly clean a badly fouled up engine is to press out the liners.
The brass drain cocks are very close to the liners and can result in them corroding through. If removed and Ok refit slightly differently orientated.
Regardless of the cooling type the exhaust injection elbow is another place that gets bunged up.
It's built like a brick outhouse so if you keep on top of cooling system problems it'll last forever.
We dropped a valve into a combustion chamber (MD11C) ... nasty expensive mess, especially as we had to keep it running .. believed to be the result of short runs not getting up to full temperature causing condensation, rusting, cracking then failure of a valve spring.
BTW Workshop manual at
http://www.bluemoment.com/downloads.html