Hey your website sure grew since the last time I visited!
If you are looking for a "Uberdruckventil" go have a look here:
www.914-6-club.de and click on "ersatz teile"
There's a message there right at the top about a Swiss firm that makes an exact replica.
It's listed for 850 swiss franks, which is about $650
Their phone# is at the bottom of the message...
I hope it's usefull for ya
cheers,
Jeroen