Tips for Endurance Racing
Over the past few years I have gotten some experience with this and people have asked me for tips.  Here is the stuff I have learned, these tips vary based on the type of race and how far away help/pits are:

Try to have a backup solution for every potential failure point. For example zip tie a spare clutch cable to the existing clutch cable, so you can do a hot swap in the field. Use zip ties, extra bolts, or weld better brackets/bracing on items that have broken before.

A basic toolkit is good to have attached to the quad for basic repairs.

Zip tie a small gatorade bottle or two full of extra gas behind the front bumper. You never know when you might need it.

On long races an air filter change might be necessary.
Run the chain a little on the tighter side to start the race. By the end of the race it will probably stretch a bit.
Double check all of the nuts and bolts prior to the race, make sure all bearings/bushings/hubs are in good shape.

Every rider should have this stuff in their camelpack
Plenty of water, co2 tire inflators, matches or lighter, flashlight, copy of race map, tow strap, powerbar, duct tape, zip ties, tire patch kit, gas syphon hose, spot gps tracker for emergency notification, cell phone or sat phone, a little bit of money in case you need to pay/bribe someone for help.

It is also a good idea to pop some energy gu during the race.  Triathletes, marathon runners, etc all use these energy packets to get a boost of energy.