Gary
If it hasn’t been in use then the most likely damage will be Black Wire Rot. You should remove and discard the rechargeable batteries (be environmentally sensitive – don’t just have a crafty look around and then bin ’em). Check the negative wires by pulling back the PVC insulation and seeing if the wire inside is still bright silver or copper coloured. If it’s black/corroded then it will need replacing along its entire length. When finished doing this, replace the batteries with modern equivalents and test the radio.
Having said all that, if you don’t feel up to the task and you don’t know a friendly electronics engineer who’ll do it for Mate’s Rates then, like Ashley and Tony say, it would be cheaper to buy a whole new set.
I had a Horizon 4 channel set back in the early 70s and it was nowt but trouble! I think that Horizon were at Ampthill in Bedfordshire, but I very much doubt if they have any connection with the ‘new’ Horizon Hobbies who import and service Spektrum radios.
Dave M