The MABDR isn't exceptionally tough and is mostly a really nice ride through State forests. There are however some water crossings and depending on the amount of rain before you go, they may be more numerous and moderately (knee depth) deep. If you were going to carry spares, it wouldn't hurt to have an extra foot peg, shifter lever and foot brake lever. One of the guys we were on the trip with on a Vee Strom broke one of each. I have folders on the Stelvio, so didn't have any issues. I would definitely have double take mirrors or similar, and would definitely have barkbusters or similar just in case of a drop because some of the area you'll be riding in is fairly remote.
I would say the number one thing though is the right tires. I always run Kenda Big Blocks on the Stelvio and they work really well in the dirt or road. They're a 50/50 tire, maybe with a bit more bias towards dirt. If the weather has been really dry before you go, the stock Pirelli's will probably be fine. If there's been some wet, you'll want knobs for a little more traction.
There are some stretches with a weird packed surface with something like pea stone on top, and you might find that a little challenging with a road biased tire. Not impossible, just not as comfortable.
It's a really nice ride, you'll love it. The Washington and Jefferson forests are gorgeous.