The crack seems to have been caused by the welding on of the aero shifter mount on top of the down tube. Must have overheated the tube and weakened it. Also looks like the frame led a very hard life, judging from its overall condition.
It will be quite a noble effort to resurrect the frame. Lots of work and expense, so be careful you do not put in more than the frame is worth, unless maybe there might be an important personal, sentimental connection you might have with it.
BTW, I think I've seen a similar head tube on an aero style Gitane frame......