charge the battery on a trickle charger 2 amp setting for 24 hrs. load test battery. If it tests good, then install it and start the atv. put a multi meter set to VDC and make sure it is charging more than 12VDC or battery voltage. If the system isn't putting out the correct VDC, then it could be the regulator or the stator not putting out. At that point, take it to a shop cause it is a little more to test the other things and if you are not sure what it is now, the rest of it may be over your head.
2001 Mountain Cat 600 EFI LE