On my td6 the main belt broke and I was still able to ride until the temperature rose to the red zone (20klms) +-. Is there any chance that something has been damaged in the engine?

It depends I guess.

The expansion tank cap incorporates a pressure relief valve and if it vented then the likelihood of damage occurring is higher as it indicated an significant increase in water temperature = increase in system pressure. Obviously with no water circulation the hot spots would be in the cylinder head area. Current: 2003 TD6 HSE. 161,000 miles
Previous 1993 Classic 3.9