I was hit by a car and one of my frames suffered a structural "buckle" It is very slight. The local frame builder suggested I replace the fork and ride it as is. He said if it did fail it would give plenty of notice and likely still be attached if the weld were to give.
The damage is very hard to see unless you trace it with your finger. Would you ride it?
Vicini road racing, Canondale canadian olympic team frame (1984) with Ofmega Mistral and other cool vintage ultralight components on it, Specialized RockHopper, Cambio Rino track bike (Gipiemme equipped), swb recumbent, Leader track frame.
I am assuming that you are talking about a very expensive bike (and not a few-hundred dollar bike in which case who cares?)
Do this simple test:
take a piece of string, and run it from one rear dropout over the head tube and back to the other dropout.
Now measure the distance between the string and your seat tube on both sides.
Is it the same? If not, try to bend it back straight (you'll have to figure out yourself how to brace it and do this), or discard the frame or have some serious rebuilding done replacing the tube and lug.
Now, if your buckle is from a frontal collision force, your downtube is crushed slightly at the front end, that is harder to answer. You may also have a very slight buckle just in front of the bottom bracket as well (assuming it is the downtube that has the buckle). If it is the case, you want to check and see if the head tube and seat tube are on the same plane (ie, if the seattube is perfectly vertical (side of bike), check if the headtube is also... this is no easy task to check, as there is room for a large margin of error in checking by sight - only an inclinometer could tell you for sure). If they are not, steering/handling could be frustrating. If they are still on the same plane, then at worse your steering has become a little more aggressive, with a steeper head tube angle. You'll have to risk the expense of a new fork to find out. But if the buckle is very slight, I'm willing to bet it will hold just fine. My concerns are in frame geometry, and handling of the bike.
I haven't seen it nor pictures, so it is a bit difficult to comment.
BTW, If you would like a nice vintage ultra-stiff bike, I have a Cannondale for sale. It was an unused leftover backup/spare frame from the canadian 1984 olympic team, as it is without decals (forbidden in the games back then) and only a couple dozen of these were made for the team. I have too many bikes, (road, track / fixie, mountain, and just bought a recumbent), and my knees and back don't forgive me anymore for serious riding or racing. The Cannondale is equipped with the lightest components available in the 90's (no not Campy S-R, lighter than that!) but not as light as the new stuff available to the pros today. It is ideally for a very strong rider 6ft2+, but can do for 6ft1 (maybe even 6ft minimum).
i wouldnt be afraid to ride it at all. if its going to break, itll let you know about i long before anything dangerous happens. im actually riding a buckled lugged steel set myself, you gotta do what cha gotta do.