Fair enough, it's probably possible to get a replacement lower cage - Shimano are pretty good at supplying the parts - might be worth a call to SJS to see if they have the right one.
Personally I'd be looking at a parts derailleur from ebay rather than replacing it with a newer 9 speed one - the Sora derailleurs are great - possibly better than the older Ultegra, but they won't match the aesthetics of your current setup.
