The mixture screw is plugged because it's not intended nor required to be adjusted post-factory, and there's no reason that calibration would "drift" over time/mileage.

Adjusting the mixture would just be trying to compensate for the actual reason it's running rich, so better to chase that down, or rebuild the carb, or if necessary, replace the carb with a rebuilt unit (if your area requires a stock carb for emissions inspection) or a Weber swap (better for simplicity, performance, fuel efficiency, and actual tailpipe emissions).