I have set out to find a replacement, but am having difficulties in the great land of non-metric parts. I have been able to find a replacement with appropriate dimensions, but it is made of plastic and not good for temperatures above 160-degrees F (71-degrees C). I found another with all the correct dimensions and temperature rating, except for one dimension......it is an M33 x 1.5 thread instead of the necessary M35-1.5 thread.
Anyway, I just wanted to see if anyone out there may have some leads on a replacement sight glass.
These are the dimensions that I've gotten for the part...

paso750 (Gerhard) has pointed out that a person is selling some on eBay out of the Netherlands, but I'm a cheap bastard and don't want to spend $50 USD plus international shipping for the part (at least not yet). The ones that I have been finding that "almost" work are in the $6-$18 USD range, so I'll pursue this a little bit longer until I give up.
Here's the auction that Gerhard found earlier...
http://cgi.ebay.de/Ducati-Pantah-Sport- ... dZViewItem