I am not sure what piece you are referring to but I have a Giro helmet and recently broke the piece that had the adjustment on the back. I looked up parts and found it fairly easy and cheap, maybe $10. Took maybe 5 min to replace once I received the piece. Should be able to fix it. I wish I could remember if I even bothered going to the Giro website. Think I Googled it and found it on Amazon or Ebay.
Yes that is the piece i am looking for but have found multiplies on the sites. Aero loc and aero loc-5 are 2 that i have found i am not sure which one works for this helmet though. As it seems it is all one piece rather than the Aero loc pieces that are only the adjustment dial .