Answer: Plums, peaches and other stone fruits are not long lived trees. You could try pruning it back in late winter and fertilizing it well through next year. A guide would be to apply one pound of a 3-1-2 ratio fertilizer (such as 15-5-10 or 16-4-9 or something similar) per inch of trunk diameter to the soil surface throughout the entire area beneath the canopy of the tree (or where the canopy would be if limb is missing on one side). Do this just prior to spring bloom and repeat in two months. Water it well but don't keep it too wet.
Regarding the idea of starting a new tree from the seed. This is usually not a good idea. The "daughter" tree will not be the same genetically as the "mother" and it will take years to grow to maturity (bear fruit). So, you may wait years to find out it is not worth having!
If you are a real gardening do-it-yourself'er, you could plant a new tree nearby and then bud or graft your old tree onto the new one. The grafts (or buds) would produce branches that bear identically to the old tree.
Thanks for the question!
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