possible that the magneto key is missing. make sure it spins with the engine.
pop off magneto and sand/clean all contact points.
black to black and frame, blue to blue, and remove killswitch...
not much else can go wrong with the things.
assume its a newer magneto with only the black and blue wires, no white wire?
sometimes the older white wire magnetos keep running if you earth the white wire...sure sign the magneto is about to die. get rid of and replace if so.
if everything in the magneto is fine, start thinking of getting a new CDI/coil...little black boxes. horrible things.