Modern Vespa

So I noticed a crack on the top left side of my headset on my GTS, starting at the opening under the mirrors, mysteriously appeared. To my knowledge there has been zero impact to that area but somehow the crack appeared anyway. So do dealers sell pre-painted headsets and if so how much are they? I guess I need to put something on the to stop the crack from growing in the meantime, what would be a good idea?

Get some JB weld at NAPA once its dry you can sand and touch up paint and be almost good as new

I've replaced one on an LX and it was more than $100, ouch.
If you want a cheap solution, try this:
http://www.scooterwest.com/item_detail/Chrome-Head-Set-Cover-GTS/873/
if you don't mind the shiny!

if you can get UHU allplast glue, that will reliably "weld" the ABS plastic

By i

To answer your question yes the dealers can get prepainted replacement headsets for you.

I've read that the brake fluid can crack the plastic. Don't you have a leak (or your mechanics change the fluid? )
To fix your problem, you can dismount the cap, and weld the slit by dissolving the plastic along this one with a soldering iron.