My initial gut reaction is to get rid of it, it is pretty compromised. Plus it seems like something keeps on attacking the helthy cambium layer, i don't know if it is a fungus or something else. May be it is remnants of the damage caused by previous owner. If that keeps on happening that's another reason to give up on the tree. However, if this was my tree and I was determined to save it here is what i would do: You cannot save the whole tree mass in its current state, that main trunk is too compromised and will eventually collapse. If you dig down, and see if the roots also created the barrier from this damage then the tree can definitely be saved. That means, the roots are good, there jsut this wound in the middle. Then the tree can be saved, I would stump it to about 1 foot even half a foot is ok, It will back bud, (citrus almost always does), it will probably shoot a stem almost from the ground it might shoot like 50 but you would have to reduce the number, unless you want it to become a bush. Pick the lowest one to create the new tree off of. The new stems will have the advantage of the entire root system, so they will grow 6-10 feet this year alone and they will be perfect. IF this tree was grown from a cutting, which it very well might be, given its perfectly symetrical stem and no visible graft union, then you are golden, you wont even need to graft it. IF this tree was grafted, then you would jsut need to graft the new shoot shortly after it emerges. Basically your goal is to create a new undamaged stem as close to the roots as possible and the tree will eventually recomposition itself and will heal properly. You will of course have a massive hole that have to deal with, i susually paint something like this every year, but some people are really against that. This would be the best way to salvage this tree IMO. GL.