The cable could be just bare copper wire, there is no need to have plastic on the cable, it is not as though it would be dangerous if it touched the chassis. Adding plastic to protect plastic seems to be a waste of money, route the cable away from excess heat and the insulation should be fine. The replacement for the OEM ground wire should be long enough to make a loop in it if possible, too short and it risks getting work hardened and slowly fracture over time.