Can I replace a Morco D61B with a Cointra water heater?

The replacement that is comparable to the Morco D61B is the 5 ltr/min Cointra version, however, it is short by one litre a second.

The flue from the Morco D61B is compatible with the Cointra 5 Ltr/Min Water Heater, so you will be able to use your existing flue with this unit.

However, you would need to purchase a G11E flue to be used with the Cointra 10 Ltr/Min version. The replacement flue for this is Morco EUP11 Gas Water Heater Flue Kit.