I well let’s start with the won’t start issue. Is there a way for you to trace the starter button on handle bar and use a multimeter to check if the switch is good? Make sure the kill switch,neutral switch, etc anything that would prevent it from starting is actually disengaged.
As for not charging does the atv have a seperate voltage regulator and rectifier or is it all one unit? Also never underestimate the problems a simple bad ground will cause.