I've worked on cars for quite a while, and I'm a sucker for a bargain. At some point I started doing maintenance and repairs on a family friend's quads and hanging out at his house all the time. Then I got a project quad of my own, one thing led to another and now I keep a few more quads on his property than he does lol
I've seen the metal tab that fires the trigger coil dropped off rotors/flywheels before. And a flake of metal that bridges the air gap stops them working too.