Plaｒepair(TM)(Plastic repair kit) is one of resin casting kit, same as "Super-bond" that is popular for dentists.
This time we are going to use these ingredients:
1) Plarepair (TM) powder (black)
This is 100% acrylic powder.
2) Plarepair(TM) liquid
This is Methyl methacrylate
3) "moulage you" (型取り君). mold plastics
It is low temperature deforming material, a kind of polyethylene resin.
3) Another old radio's knob for a prototype
4) A new general-purpose knob
It should be about 5mm smaller than prototype knob, and the hole diameter must be match the shaft of VR or rotary switch.
You can buy Plarepair here
. And perhaps there are other products similar to it in your country.