Weird that the crack runs lengthwise along the tube, much more typical for cracks to propagate around the circumference tube. Due to orientation of the crack, I think it would be a reasonable chance of success to repair the existing cracked downtube by drilling small holes at ends of the crack to halt propagation, notch along length of the crack with a cut-off grinder wheel, lay a TIG bead down in the notch, and then grind/file/sand the surface back to flat. Such a TIG crack repair could be done much cheaper than attempting to replace the entire downtube by un-brazing from the lugs.