I've been giving this a lot of thought, and my needs are far different that the folks out in real remote regions. Here in the midwestern states, I doubt I would be out of CB radio or Cellphone range in most areas that I would travel, I can't imagine anyone being lost around here for multiple days but it is reasonable to assume a breakdown would yeild one very uncomfortable night and maybe a bit longer. So my plan is for a pack that will fit under the right side passenger seat and will be divided into a tool kit for repairs and a modest survival kit.
In the survival kit: 3 MREs, 6 granola/energy bars, Beef Jerky, first aid kit, 3 100% wool Swiss Army surplus blankets, 3 space blankets, 9 chemical hand warmers, 6 chemical light sticks, water proof matches, spark stick, 2 LED flashlights, fresh alkaline batteries each season, 2 hunting knives, folding saw, water, 3 flares. Duct tape. Toilet paper. Candles (remove humidity from the air and create some heat). The point is really to be comfortable and safe for a reasonably short period, hopefully inside the cabin. This is not much more than would be carried in my SUV in the winter but in this area I think it is sufficient.
In the tool kit, are basic tools. The Snow Trac is a very simple vehicle. Basic wrenches, screwdrivers, nut drivers, pliers are all that is really necessary to maintain these. A very sharp knife, like a carpet knife, would help with track repair. The 'variator belt' is probably the area of most concern, it controls the power to the tracks, an extra belt would be necessary in remote areas, I don't have on, I plan to get one. A fan belt. Some extra belting for the tracks would be excellent to carry, I'm planning on making a 24" run of track that could be cut into place as a repair. A good snow shovel, maybe 2 of them! 2 - 2"x30' tow straps.