As alternatives to the solutions already offered, I had a similar situation last summer, but after some internet searching, I found some brass and copper pop rivets normally used for metal roofing. They weren't cheap, but I found a company that sold small quantities and the shorter ones worked just fine without impeding the steering tube. Even the longer ones could be whacked down if they extended too far.
http://www.grantlogancopper.com/
If you go the rivet route, it's best to use a punch with a concave head that fits over the rivet head. This helps put the rivet in straight and doesn't mangle the head. You can find these at most hardware type retailers sometimes sold as setting punches.
http://www.lowes.com/
In a pinch, I've also used small brass nails, cut down so they don't stick too far in. Glue the headbadge on with epoxy and place the nails into the holes so it looks like the headbadge was riveted on.
Fred Durrette Summerville, SC
To: classicrendezvous@bikelist.org Sent: Sat, June 5, 2010 11:02:34 AM Subject: [CR] Mounting Headbadge
I need to mount a brass headbadge to my 1950 Wilier - does it get screwed or riveted - What is the best way and how does one do this? I think I've done just about everything else to a bike but stick a headbadge on one...
Suggestions?
Thanks
Rob Goughary
Stamford, CT USA