I recently had quite an adventure trying to secure the headbadge to my 1970 Motobecane Grand Touring. I must have bought about 6 different types of self-tapping screws (100 per bag) until I finally asked my LBS owner, who is also possessed by old bike demons, who told me they are generally stuck on with a variety of things. So I filled it with Loctite silicone and put in two of the self-tapping screws that I had ground the ends off of (to prevent them from hitting the steerer) and that baby isn't going anywhere;-) What a pita is was though, only to find such a simple solution.
P.S. Darn I thought I would have this bike finished for this weekends vintage bike ride in L.A. or Pasadina. Oh well