No .. can not simply replace with normal coax .. the coax used is a litz coax the internal and shield ..
also the coax is of low capacitance .. so is quite special ..Then if you do find the correct type cable it is a big job breaking into the coil to fit ....
If the outer protective sheath is decomposing ..for a bandaid fix ..just try taping it with electrical tape .. have done this in the past ..
otherwise is a replacement coil is the way to go..