Possible Fix: Do as the previous person replied and drill a very small hole in the case to stop the crack from spreading. Remove the pivot bolt and check to see if it has gone completely through. If not and you feel like keeping it around for a while longer use a V-shaped file and file the crack out. Then (if you can find them) use a special aluminun repair welding rod (usually can find them at auto or boat shows. Seller welds coke cans together with hand held bottle torch for demonstration) Fill grove with welding rod and file smooth.

Reality Fix: Avoid all the trouble of time consuming work of repairing as mentioned above and see if you can purchase this part. You could even step down to another level of Campy 9-speed derailleur. There should be no issue since it's the shifter that has the indexed points and not the derailleur. The derailleur is just spring loaded and does what the shifter tells it to. I'm sure the replacement part is not cheap and if your part is 3+ years old I would just change it due to that.

As for those alum rods. I used them on a 1968 Johnson outboard motor to weld 3 complete breaks in a bracket that held a belt driven generator for battery charging. Not only did it hold, it lasted me 3 years and the guy I sold the boat to 2 years ago said it's still holding. Hey, it's worth a try.

Good Luck!