Hi
I believe the 1971cc petrol engine in the Ducato 1/Citroen C25/Talbot Express was based on a design by the PSA group (Peugeot/Citroen). Versions of this design (referred to as 106 or XN1) were used in the Peugeot 504 and 505 cars from about 1970 to 1990.
The exhaust (outlet) valve Original Equipment Numbers (OEN) are 0949.21 and (Fiat) 7 910 028 074
Versions are made by:
Mahle 031 VA 30597 100
AE V34460
SASIC 9490550
Intervalves AG 1798.036
TRW 0949.21
Unfortunately I haven't found any suppliers with new stock (which is probably where you have already got to !), so you might be forced to look for a secondhand valve or a head with valves fitted.
You could maybe try a classic Peugeot specialist like SFM automotive in Halifax, though I have no idea how good they are.
Good Luck !