[morzh]

If you have major problem with wire insulation falling apart due to it being guttapercha (rubber) crumbling away due to drying, unless a particular wire is just too difficult to replace due to it coming out of a coil or otherwise not well accessible, usually the advised course of action is full replacement.
I made ampn exception for white (beige?) wire n my radio, it was as flexible as it ever was, different color rubber ages differently, black and red being the worst in AK radios, but otherwise I replaced almost all of it.

Heatshrink is not really an adequate overall solution. Spot fixing is ok now and then, but bad wiring, if at all possible, should be replaced.