Hi, well the bike wont disintegrate all of the sudden with a crack like that, but I have seen happening is that the crack will get bigger as the time goes by, since you have not used the bike in a while probably the crack has been always there and never noticed it.
That crack could be because of lack on installation, facing is always good when installing a headset, but you have to add the fact that the bike might be old and the lug simply gave up, the other thing is that many times with series production frames like that one (lug work tells) the tubes in this case the front and the top tube (I believe) are not touching to each other 100% so the lug have to work even more to make it up for the structural fail, If the lug failed the front tube failed as well.
As for repair, u can change the lug or braze the crack from the inside out, sand... and that's it, good as new. The advantage in brazing the crack again (this is done with broken dropouts) is that you wont mess with the angles no more, probably the only thing you will need to do after wards is just face the lug really well. Not a builder but i have seen my master builder done this several times.
Good luck and you have to tell us how to chrome at home
THat looks interesting
