Use a reed switch.

https://www.sparkfun.com/products/8642

GI~ -----> Reed Switch -----> LED Bulb -----> GI~

Solder a wire from one side of the wiring GI to one side of the reed switch. Solder the other side of the reed switch to one side of an LED bulb (preferably an AC/GI bulb) then solder the other end of the bulb to the other side of the GI wiring. (You can just tap off a GI bulb somewhere near the magnet).

Zip tie the reed switch to the magnet. Each time the magnet fires, the reed switch will close and that will complete the circuit, lighting the bulb.

If you want to get really fancy you can buy a PIG II from www.dkpinball.com and make the light fade, pulse, blink, flash, blow a horn, whatever. But it's a bit expensive for a single mod.