Tristan,
My first lead fizzled, but with Google Fu I found the likely manufacturer.
Carat Medailles Inc. in Montreal 514 937-5665
The kind fellow there says they do repairs.
The base metal is 85% copper with a nickel overplate
and a 14K or 18K gold plating over that.
!4K or 18K gold plating is actually a gold nickel alloy.
Color matching may be difficult if you go somewhere else
My suggestion is that they are the most qualified to do your repair.