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Lemon trees have a lifespan of, roughly, 50 years. This kind of damage will be detrimental to the old tree. It may never produce throughout the rest of the trees life, but there is a chance that you can still get a few blooms. It may take a year to two for recovery, but it is possible to see, at least, something from the tree.

The best thing to do from here will be to remove the old tree and stump in trade for a fresh tree, but keeping the old tree for the rest of its lifespan isn't a far-fetched idea.

This article will help you to grow a new tree if you decide to plant one: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

We do not sell anything here at GKH. We only offer information on growing. We don't really recommend particular vendors either, but you might try places like Amazon, Etsy, Ebay.

In the meantime, this article will give you information on the tree: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

If these are small and yellow, then it is likely that they are being killed off by the tree, and not ripening. This is due to a lack of phosphorus and potassium, and sometimes lack of light.

A citrus fruit takes about 9 months to ripen after pollination. If it has been not even close to this time then it is likely that the lemons are killing themselves off.

The same will be true for the fig. You will not need to thin the herd, as the tree will do this on its own. I would recommend testing the soil and finding which deficiencies that you have. Once you correct these, the trees will produce as they should.

Without seeing the damage, and just by description alone- It sounds like overwatering may have led to fungal infection.

Unfortunately, this can have many causes and depending on the environment- whether the tree is indoors, or out, whether it is container or in ground, the temperatures around the tree, feeding and watering habits, and even much more.

This article will help you to care for Lemon Trees: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

There can be several reasons for leaf drop.
This article will help you.

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaf-drop.htm

Hard pruning is detrimental. It is extremely stressful for the plant physiology as well as you probably cut off all the flower buds. If the plant survives, which is not a sure thing, then you will have to wait for it to refoliate and bloom again. It could take a year or two. Hopefully you have not exposed the woody stems to direct SoCal intense summer sunlight and sunscald of the bark and underlying tissues. That can set it up for wood decay and structural collapse. The only thing you can do is to water and fertilize and wait and see