Basically toast I'm afraid. The crack could be TIG welded up and then the ST reamed, but as Kontact says, the problem is that brazed-on collar. The braze will melt into the weld and ruin your day. So you would want to try and remove that collar completely first. But that's going to be difficult as half of it is on a piece of metal that is already trying to fall off. If you could get the collar off (grinding/sanding is how I would tackle it) then you could reassess the situation and see how much good metal you had. It might be possible to squeeze it all back together around a seatpost, insert a new patch piece in the slot, weld it all up, recut the slot, ream it, and then use a collar that just sits on top. It would be fun to try to see what happened but I wouldn't have high hopes.