i know that for the 550s, the length of the fork legs changed. the tarozzi only fits the later models with the taller fork legs, not the earlier ones. i'm not sure about the 550 but it would make sense if it had also shorter legs like the early 550s.
when i looked, i could not find a commercially made fork brace for my 550f1. eventually i had one made to my measurements by tkat:
http://www.tkat.com/forkbrace.html i can look up the specs and share them with you if that's of any help.
there is, indeed, a brace built into the front fender but an additional one does help to steady the fork which on its own, is somewhat flimsy.