First of all clean the area around the trees. Everything must go. By "everything" I mean dead fruit trees, forest growth, shrubs, undergrowth etc.
Second, the soil by now will be so depleted it will need fertilizing, but avoid the temptation of overfertilizing, as an excess of nitrogen will produce spindly, weak growth. Use either manure or a mineral fertilizer, applied at the end of the Winter when the soil begins to thaw.
Third, those trees will need to be re-trained. This is the area you'll be spending the most time and effort into. First try understanding what shape the trees were originally trained into: given the age is well possible they were originally trained as standard or half-standard trees, which isn't much used nowadays. The trees need to be pruned during Winter while they are dormant and regardless of cultivar is very likely they'll need spur thinning.
If the trees require balance correction, you'll need to carry out nicking and/or notching, which are done in early Spring as the tree starts vegetating again.
This alone should be enough to get the orchard restarted, but mind pruning alone will be quite the task and mind you'll only be starting.
Pollinators may have died out and, given this orchard's age, you may even get triploid cultivars with a single pollinator left. You may also have trees requiring bark-ringing, which is easier said than done, and as the rootstocks are highly likely to be pre-Malling and are surely pre-Bugadovski, they may have survived the graft and be the ones giving fruit now. These fruits are obviously edible but usually poor-tasting.
In short you have a Hell of a job ahead of you.
