I'll be on EXO's Baja run in May and I've done many runs with a mostly stock setup. The only difference between stock and headily mod'd is how fast you go when it gets rough. You'll be fine for sure. The stock trucks tend to break down less. Probably because they aren't running as hard, but still.
To expand on what
@RivRaptor said:
You want two spares, with at least one spare in the bed. I wouldn't go without both in the bed, but it is a nightmare to get a spare from under the bed when buried in the sand.
VHF Radio is critical. A hand held is nice to have for when you stop and are out of the truck, but you need a high powered one with an antenna. To avoid dust, you could be a couple miles from the next truck.
An off-road GPS. Most people use Lowrance, but I've seen a lot of iPad or other tablet style setups getting more and more popular.
In addition to the belt, I always bring a couple extra air filters and a cooler of beer. Those are cheap, lightweight parts that frequently need replaced. The next most likely to go out would be the power steering pump, especially if you run tires heavier than stock. Extra fuel is kind of a no-brainer, especially with a supercharger.
That is the end of required stuff. Here are a couple suggestions I have in addition to that.
As part of your light setup, don't forget chase/dust/rear-facing lights. Generally amber or red, but any rear-facing off-road light will do. It is so critical if you are running in a group. I promise, it'll get more use than the thousands you can spend on forward lights.
Frame reinforcement and bump stops. Everyone (SVC, RPG, ICON, etc., etc., etc.) makes a kit for a reason. It is an easy to install/remove insurance policy against frame bending. If you bend a frame with one of their kits, you were going about 4x the speed you should have been going and I promise other things on your truck are also broken.
The factory skid plate sits so far forward and is so flimsy, it is almost impossible not to trash it. Sure they are cheap'ish to replace, but I would get an upgraded bumper with a much heavier skid plate. I know you said you didn't want to cut the body, but this is the frame and your truck is out of warranty anyway. I have a frame-cut bumper that has been on many, many runs over 7 years without needing it or the skid plate replaced. I was on my third factory skid plate after 2 years. Like me, maybe trash the factory one first, then upgrade and plan on leaving the bumper on the truck when you sell it.
People make a big deal about air, but as long as you aren't running lower than ~22 psi, you can drive it all the way home that way. Sure, you may not have even tire wear, but if you are running in Baja on those tires you probably aren't as concerned about even tire wear. Don't get me wrong, I carry a CO2 tank to air back up for the highway at the end of a run, it just isn't as critical as so many other things.
Recovery gear is critical for the group to have, not necessarily you. Make sure someone in the group has a few recovery straps, shackles, and traction boards. I know almost no one that has a winch in the desert. They are heavy and there isn't anything except other trucks to attach to, so a recovery strap is almost always as good.
