FWIW, I wouldn't be throwing a leg over any bike with a crack in the frame. This after I broke my collarbone when I came down on a smooth, straight road, for no apparent reason - and subsequently discovered a hairline crack in the head-tube of my aluminium frame.
Also, FWIW, have subsequently been told (by a frame maker) that it can't be repaired because the aluminium is too thin (see above ...), and only solution would be to replace the head tube - which the manufacturer has offered to do for a tidy sum (no guarantee in this case).