Carry an extra reed set, enough wire to pull springs, a closed in wrench to change main jets, a main jet wrench, extra jets for higher and lower altitudes, a short piece of wire to short out the thermister in order to check to see if it has gone bad (when the thermister goes out it opens up the circuit), a star wrench to disconnect the high beam switch, an extra VES gasket, an extra VES diaphragm, extra new plugs, a pair of long 9/16" closed in wrenches to readjust track tension, 1/2" socket to retighten the rail bolt just in front of the front idlers, a 1/2" tow rope and a second rope to run between the ski loops, duct tape, 3/8" socket to change reeds, a broken in extra belt or two, an extra bottle that the coolant bottle tube runs into to catch the overflow and then sucks it back in, and an extra CDI box.
If anything else goes wrong that this list won't cover, you are going to need to be towed out.
I also like to upgrade the spark plug caps to the waterproof PWC plug caps, add a 1/2" threaded rod down the middle of the steering column and epoxied into place, setting the front rail straps at the 1 1/2 position (which requires drilling new holes), replacing the rear idlers every 1000 miles, keeping driveshaft axle bearings well greased, adding holz scratchers, add an SLP single pipe for an extra 10 hp, and upgrading to SLP powder pros for long tracks.