it depends on how much you want to spend, having lugs replaced can be expensive, then respray cost on top of that. I have seen cracked lugs that have had their cracks filled with silver solder of brazed up that have stood the test of time. This would be a cheaper alternative, but then i haven't seen the extent of the crack in your frame.
Ask a frame builder (hopefully the good Mr. Walker or Thylacine will weigh in here) for a competent answer. In my non-expert opinion, however, I'm inclined to say do a light weld along the crack and cover it up as best you can.
Pro: the top tube is one of the least stressed tubes in a diamond frame bicycle.
Con: Tube joints are the most stressed area on any tube.
Did the frame come like this? Is there a chance that this damage occurred in a wreck? If so, then it's quite likely that simply brazing or welding it will be sufficient to get it to hold together even if it's no longer raceworthy.
Hmmm.. good solid 531 shouldn't break easily, including street riding. Is there a chance that it's not 531? If it's an old high-end frame, it COULD be 753, though that's pretty rare and not quite period-correct. 753 is pretty delicate, though.
I would say perhaps the tube has rusted out, but I don't believe lugged construction needs holes at tube junctions the way welded does, so there shouldn't be any way for water to get inside the tube in the first place. It shoudl be 100% virgin 70s air.
But I could be wrong.
I see that the crack seems to line up very nicely with the furthest inward extent of the lug shaping and I wonder if there is just too much flexing in the frame which gets expressed right at the point before the lug adds sufficient rigidity to dampen any up-stream flex. I believe this could be true if the tube wasn't properly mitered when fitted to the head tube.
Ultimately, this is all non-expert speculation though. Given that it's happened while you've owned the bike and is presumably the result of your riding, I would consult a frame builder.
It's also true that everything eventually wears out. This could be your frame's time to go.
As an amateur framebulider, I'd say that unless you're willing to spend about 4-500 dollars, your frame is done. That bike needs a new top tube and upper head lug at a minimum. I don't know much about repair (having only done one headtube replacement and one rear triangle rebuild) - *I* would also replace the headtube, because that would make my job easier.
Seeing as how the bike is from the 70's - from an unknown maker - I would start shopping for a new one.
I would agree with BT's assessment of poor mitering being the death of that lug. In my limited experience, I have not seen a frame start to fail in the middle of the lug. Keep in mind though - 531, 753, 69000 - whatever - that frame is 30 years old - nothing is unbreakable.
If you're strapped, you could *TRY* and do a ghetto braze - but the longevity of a repair like that is dubious. I would strongly advise against it in such a critical area.
as an amateur framebuilder, i can tell you that those lugs are also stamped, not cast. stamped=formed out of a sheet of metal, and welded along the seam, which somet yields an non-uniform lug inside surface, which brass won't flow nicely through. as a repair i wouldn't recommend it unless you had a friend willing to gain some experience repairing it.
as an aside, this failure has little to nothing to do with the tubing of the bike. it's a matter of poor lug stamping/finishing, and/or poor brass penetration.