This pub might help:

Here is a link that might be useful: Pruning to Restore an Old, Neglected Apple Tree

Is there any vigorous growth going on or is the tree truly in senescence with not much life and canopy happening? If there is vigor then we can talk, if not there's little you can do besides giving it some nitrogen and water to see if it rebounds.

Another option would be to cut off some scion wood and have it grafted onto modern rootstock.... This way - you can at least keep the tree - even if the old one dies...

Thanks