Old non-electric fences can be made to last for many more years by attaching offset brackets with an electrified wire on one or both sides of the fence.
Attach single offset wires at two thirds the height of the animal to be controlled. If sheep and cattle are in the same area it is better to use two offset wires (one for sheep, one for cows). However a single wire three quarters the height of the sheep will still protect the fence from both animal types.
If the old fence is tangled or has broken wires, it will need to be tidied up. Otherwise you will run the risk of loose wires causing accidental shorting on the offset wire. Remove the worst wires and tighten the others where possible.