I'm an airline pilot who frequently flies in the northern latitudes.  I'm wondering what data should be monitored so that I can fly at a lower latitude or altitude when required.  Thanks.

Umm, I guess it depends on the reason, given you already fly up north. The SWPC does have an aviation dashboard, https://www.swpc.noaa.gov/communities/aviation-community-dashboard.

Is there somewhere I could find the dates of previous S2 and higher events?

Most can be found here, https://www.spaceweatherlive.com/en/solar-activity/top-solar-radiation-storms.html, and you can pick a solar cycle. A few are missing due to the 25 event limit on there.