PARTS
I used to carry parts such as radiator hoses, belts, but never carried spare alternator or thermostat or for that matter spark plugs. You can run without the thermostat for a ways, on the older cars you could run on battery for a ways, and you can run on partial coil packs. If you are going to change thermostats, you need a BIG bucket to catch the coolant in.
I do carry a spare spark plug on my Yamaha Raptor because it only has one cylinder
And...where I get caught with a Raptor engine not running might be 70 miles off road

On my Raptor, I carry
Mission Critical drinking water
Now to summarize, I am positive my belts and hoses are in good condition as well as my belt tensioner before I leave so
I do not carry extra belts or hoses.
My
cell phone is charged up. My
Garmin will tell me how far I have to go to get parts and my
iPod Touch will let me log on to this forum for
Solutions or Ebay for
parts as well as
weather
http://www.zr1.net/forum/showthread....7041#post87041
TOOLS
I do carry set of tools including all metric sizes flex ratchet combination wrenches, complete set of 3/8 inch drive metric sockets, and complete set of metric allen heads. I include a set of good screw drivers and a couple of channellocks. I carry these tools to make sure all my bolts and screws are tight and for the special case where I limp into town and visit O'Reilly's for an alternator, water pump, or coil pack
All the tools fit in the storage compartment behind the passenger seat.
I do carry jumper cables and a digital ohm meter as well as this list of codes
http://www.zr1.net/forum/showthread....2722#post12722
The cables and ohm meter fit in the storage compartment behind the drivers seat. The Codes list I place inside the operations manual in the glove box.