It sounds like your ignition light wire might be shorting out somewhere along its length.
Try disconnecting the wire at the generator and see if the light still comes on when you switch the ignition on. If it comes on it's shorting somewhere. I'm not certain that will work with a positive earth system but it should't cause any damage if you try it.
Some useful info here -
https://www.howacarworks.com/ignitio...-warning-light