[Arran]

I haven't tried doing this myself yet but I think that you may have two options. One is to harvest a similar yoke from a part's set but the plastic form in it may be deteriorating as well. The second is to replicate the deteriorating plastic parts out of new material, someone on another forum did this with a late 50s RCA yoke that was falling apart by making a new form out of wood, I suppose you could do the same if you had some thick plastic sheeting laying around too. Silicone won't help, its like duct tape the only thing it will do is make a mess and not really repair it over the long term.
Regards
Arran