Fixing the rectifier only gives you a whistle. Not useful if you run modern stuff and want a bell. You can just make an external bell/whistle controller and forget the rectifier in the KW, described here
www.jcstudiosinc.com/BlogShowT...=412&categoryId=
can be automated described here,with a pic of the diodes used and mounted
www.jcstudiosinc.com/OGaugeAut...AndWhistleController
Dale H