It is likely that the plug needs replacing. Yes, the idle screw is the one that you can see the slot of inside the throttle linkage. Nothing needs to be loosened to allow turning this screw, but wait until you get it running before touching that adjustment. In all likelihood, once the plug is changed to a good one, the idle adjustment will be good while the high speed may need to be adjusted for peak speed depending on the differences in the atmospheric conditions from when it was last run and now.