Options:
1. Get it welded. This seems to be the quickest/most popular route and experts agree that if repaired correctly it won't give further problems.
2. Get it replaced. Ducati issued a check/recall on the 900SS for a known issue with frames cracking around the headstock. There have been a few reported cases here. Some have managed to get a replacement frame from Ducati UK foc even though the bike is out of warranty by a good ten years plus, so it may be worth contacting them.
Lots of threads about it if you search:
http://www.ducatisportingclub.com/sh...ad.php?t=51544 http://www.ducatisportingclub.com/sh...ad.php?t=25806 http://www.ducatisportingclub.com/sh...ad.php?t=23540 http://www.ducatisportingclub.com/sh...ad.php?t=19640 http://www.ducatisportingclub.com/sh...ad.php?t=17140