You might want to try to figure out why those wires got so charred before you cover things up. If they're in the vicinity of the driver-side headlamp connector, you've probably got issues with the three-wire connector on the headlamp pigtail. Or maybe a PO tried to install aftermarket fog or driving lights and made a mess of it (as in, short circuit).
If the region of the harness that needs re-covering is relatively 'fat' (say, 1/4" diameter or more), the correct way to do the job will be by using loom tape ('wiring loom'). It's just like electrical tape but has no adhesive. Buy a roll that's about 3/4" width. It's inexpensive. Takes a bit of practice to learn how to apply it. There are a few on-line articles explaining the technique (try an MG or old Ford club website). Just don't use electrical tape.. It turns into a gooey mess after it's been exposed to engine compartment heat for a while.
Nissan used thin, flexible black 'plastic' sheathing as the covering at the end of some of the skinny branches of the engine wiring harness. That sheathing turns rock hard with age and usually significant amounts have cracked and split away. Replacement sheathing is available in three or four different diameters from one of the specialty wiring connector suppliers (Vintage Connections or Eastern Beaver... can't remember which one).
If you're not concerned about restoring the harness to OE specs, there are a variety of more modern wire sheathing solutions available.