Weirdly, I have my own Alien ACX that needs this (and the headtube) doing.
Haven't got around to it yet:
Yes, I've welded or replaced many a head tube (and other tubes) on various aluminium frames. Mostly Manitous, but also Pace, Saracen, Marin, Sintesi, Mongoose, Koa and one Nishiki...basically anything 7005 can be fairly easily repaired.
Here, the whole top section of the seat tube needs replacing. I'd remove the old tube back to the original top tube mitre, leaving the seatstay mount untouched. Then splice in a new seat tube section above the chainstay mounting point, with an internal sleeve, leaving enough length inside for a long seatpost, then finish flush with the original surface.
And ream and finish the headtube to spec if it hasn't cracked already :-(
All the best,