I've repaired barrels in this condition with some success. It may be possible to remove the old glue, but it depends upon what sort of glue it is. If a hide glue has been used it can be removed with hot water, if it is cyano-acrylate (superglue) then acetone may work. If it is epoxy then I have on idea how you might remove it. With the crack cleaned as much as possible, fit the rings to stabilize the barrel. Seal the bore using a dowel wrapped with plumbers teflon tape until it is a snug fit in the bore.
If the crack is large enough, dust baking soda into the crack, taking care to clean off any that settles on the body of the barrel. If it's not large enough then skip the baking soda. Using something like a toothpick, introduce extremely low viscosity superglue into the crack, taking care to avoid getting it anywhere other than the crack. The baking soda will absorb the superglue and ensure a hard, dense filling. The crack with probably need several applications of superglue, allowing each layer to dry before applying the next.
While there is still a little depth to the crack pack it loosely with grenadilla dust (make it yourself using a nailfile and an old piece of grenadilla) and then apply the last layer of superglue. At this point check the cracks under the rings from the inside of the tenon recess and apply superglue as described. At this point you can remove the dowel. When it has set file and sand the ridge down to the level of the body of the barrel. Done properly and finished with care this repair will be almost undetectable.
Low viscosity superglue is available through music repair materials suppliers or specialist tool or bearing shops, at least it is here in Australia. You may have to your own research here.
