Improving the structure and vigor of such mature trees will be your first concern. If they are badly overgrown, this must be gone about in easy stages or, instead of fruiting wood, your pruning will produce chiefly sucker growths. Besides repairing wounds and cavities as best you can, your first efforts should focus on removing deadwood, stubs, and obviously extraneous whole branches. Leave your thinning and shaping of sound branches until you have seen the trees through one summer and autumn. Meantime break up the ground around them, feed them, mulch them for the winter, and get ready to spray in earnest the following spring.
Fruit trees tired and neglected beyond a point can never be brought back into full bearing, but that point can be surprisingly far along in their lives. And even if you fail to revive its yield after a couple of years of trying, an old fruit tree will reward you with blossoms and shade until you decide to replace it.
Two factors have basic bearing on fruit production. One is chemical. All tree fruits grow best in soil just slightly acid, from pH 5.5 to 6.5. (Some bush fruits, such as blueberries, require pH 4.8 to 5.0.) Testing your soil's acidity and adjusting it with lime or aluminum sulfate is thus a must for fruit culture.