Everything! I am over weight (the vehicle that is for AA etc) I am my own most of what I carry has been needed in by many years of breaking down.
starter motor
dynamo
generator regulator panel
complete carb + extra diaphragms, jets etc
set of 6 brake hoses
brake pipe + flanging device
ignition coil
distributor
spare: rotor, condenser, carbon brush, points, distr cap
fuel pump
set of plugs
set of ignition cables
track rod ends
accelerator linkages & connectors
power distribution thermal relays
starter switch + light switch assembly
thermostat
thermostat sensor
dipstick
rad cap
oil filler cap
head gasket
tyre valves & caps
sealant & gunge
compression tester
multimeter
megger
continuity bulb on leads
soldering iron + solder (so easy to forget that)
wire
all bulbs
all fuses
all relays
horn
horn switch
insulating tape
gaffer tape
water
WD40
carb solvent spray
brake fluid
engine oil
gear oil
GP grease
copper grease
2 CO2 fire extinguishers (one each end of vehicle)
MOD intervehicle jump lead
misc lumps & blocks of wood
vehicle jack
2 x groundsheets
2x 3-tier boxes of tools
large hammer
wheel brace
starting handle (actually easy on a pig)
selection of nuts, bolts, cable ties, rope & string
selection of rubber radiator hose + jubilee clips
user handbook with circuit diagram
vinyl gloves (not latex as brake fluid & fuel dissolve them)
mobile phone + charger
first aid kit
lots of bog paper
NO spare wheel - pig is runflat
wire coathanger
The coat hanger is partiulary useful for fishing out things from behind a hot engine etc & useful to bend up to support a droopy pipe