.... so in summary
1. leave it as is. chances are crack will continue to develop but IMO (and I may be wrong), there will not be a catastophic failure of the head tube. Worst case scenario is that the lower cup gets loose, starts to spin thus wearing the head tube resulting in a floppy lower race, and that isn't going to happen overnight
2. drill hole and fit the taller cup, the hole should prevent the crack propagating and hopefully the tube exerts strong enough grip to hold the cup in place. unless you have drill press where you can clamp the frame, a 1 mm hole will be hard to drill without breaking the drill bit - you best bet is to either go for a larger drill 2 or 3 mm or use a small electric PCB drill
3. as above but use lumiweld/technoweld (an aluminium solder) .... but i've not had luck with this stuff so i don't think it will hold
4.. repair the tube by welding it - someone ^^^^^ already offered to do that so it would cost you return postage plus a monetary gift for the welder. NB if you are local, I could have a go with my AC TIG but not really used it much so I'm probably as useful as Mike Tyson on mastermind
5. have the head tube replaced(most expensive option)