[Arran]

One thing that nobody has mentioned is that there are actually two windings in the loop antenna coil, one acts as the antenna to the set, the other is to connect an external antenna. I can't remember the ratio but I think it's 3:1, they may give a DC resistance of each coil in the service literature. I think that the best course of action is to make a form out of some plywood, or lumber, by tracing out the inside of the cabinet, and cutting out the shape, including the ribs that the back attaches to, but you will have to make it smaller to accommodate the wire, this is if you want to make a facsimile of the original loop antenna. Otherwise you could just make a electrically similar coil and mount it to a replacement back, which is likely the easier route.
Regards
Arran