Charging system not charging like it should.

I have a 1974 RD 250 and the charging system is not charging the battery and has a hard time keeping the lights going while the battery is charging. When it is running and go's to idle you can see the lights dim and the blinkers will stop working. Is there a way to fix that or is there some kind of new system that I can put on it that will adapt to it. please let me know.