Before the 1940's, most electric and electronic devices were made with cloth covered wire. This material usually stands up fairly well and very often a radio made with this will require very little replacement of wiring to be safe. The exceptions are if the wire has been exposed to excessive (very excessive) heat, eaten by rodents, or exposed to insect infestation (yuck).
During the mid 1950's, all manufacturers started using vinyl insulated wire. This is practically indestructible unless burnt or occasionally eaten (yes - sometimes vinyl insulation does get eaten).
Between these two dates, many manufacturers used rubber insulated wire. This stuff stinks. It nearly always either rots or becomes friable and large portions of it MUST be replaced in order to allow the radio to be repaired and used safely. I have had a nice little RCA Master radio which actually worked reasonably well but required almost total replacement of all its wiring. The insulation literally crumbled to the touch. For safety reasons, a radio in this condition cannot be left alone, and I actually either replaced or covered in heat shrink tubing almost every wire in the thing. (picture here). In this case, time and material charges must be made due to the incredible amount of time and effort required to replace unsafe wiring.
Starting in the late 1950's, circuit boards began to be used, particularly in the most inexpensive radios. There is very little that can be done to repair a bad circuit board. If there are a few bad traces they can be bridged but the resulting appearance may not be very good. A physically broken or burned board is NOT SAFELY REPAIRABLE even if it can be coaxed to work. Fortunately, most circuit boards are in good condition and few need any type of repair at all.