A comprehensive guide to the old magazines may be found here:
https://bg.battletech.com/forums/index.php?topic=437.msg6457#msg6457Of those, one of my favorites was "Tricks of the Trade" by Mike Speca in Dragon Magazine 166, which introduced home rules for what we know refer to as "Quirks" for individual 'Mech designs, based on the fluff. Dragon 161-163 had some bare-bones campaign rules, though nothing close to what you'll find in Campaign Operations.
StarDate Vol. 1 Issue 5/6 also had a fascinating "History of the Inner Sphere" that shows a first draft of the BattleTech universe that diverged wildly from what we have now (for example, the Rim Worlds Republic was one of the Inner Sphere states, bordering the Hegemony, and its slot in the Periphery was taken by the Rift Republic). Stardate Vol. 3 #6 had an interesting NAIS MechWarrior Training Manual in which Blaine Pardoe writes about battlefield repair theories, the utility of different formations (based on real world tank tactics), cockpit layouts, and quick-start time requirements.