The 31mm Clutch piston (#067.016.460) is very hard to find, I've just spent 2 months trawling the usual suspects to no avail, until I hit up Back to Classics (
http://backtoclassics.nl) who had the part in stock, and it's in post to me.
Whether they have another one I don't know.
However I did just receive an email from Old Racing Spare Parts who do have the item in stock, contact via
info@oldracingspareparts.com.
Cheers,
John
(If you're ever in Sydney you owe me a beer!)