Hello to Sweden!
The right diameter should be 180mm unless anybody else has changed it. One more important thing is that there are two types of clutch release bearings. If everything is in the original state you should have the new version of the clutch with the new type of realease bearing: viewtopic.php?f=2&t=6006&p=65085&hilit=353+drucklager#p65085
This new version is called "Zentralgeführtes Drucklager"
Hope this helps, there will be some offers in the internet.
NB: The spline profile is exactly the same as Volskswagen Beetle