It turned out the part is still readily available. I think I was mistaken about the part number. 77's are probably Renault numbers, so I assume it is used in other cars too.
The first of the photos below show the two very small cracks in the old pot. The next one shows the old one with hoses attached. Then the new one, and finally the installed swirl pot.
She now has a water and airtight cooling system again