Alright...
That light depends on a couple connections to operate.
First is a good connection to the start switch, so it goes negative when the switch is turned on (assuming you are still positive ground). So, double check with a test light that the white wire goes to battery positive with the ignition switch on.
Next is a good ground to the “E” post on the regulator, so test that E is positive at all times.
If those are good tests, then the most likely cause is the cutout is not “cut out”. Pull the cap on the control box and look for the 2 point contactors. One should be closed (the voltage regulating contact) and the other should be open (the cutout). Of course this is with the engine off, and the ignition both on or off. If the engine is running, then one contractor should stay closed at all times (cutout contact), and the other will cycle (regulator contact).
If you find the cutout contact closed, it may have welded shut, so pop it open and give it a good cleaning with a file and contact cleaner. If it is open and the ignition light is still not lit with the switch on and engine off...Then...
Ensure the yellow wire to the generator is intact and shows a connection to battery positive with some resistance (6-20 ohms approx).
If none of that works...make sure the bulb is good...or maybe check that first! Like I described above, these bulbs have a dab of lead at the tip that deforms and loses contact over time.