I use these Exhaustion rules instead of the ones from 2014 edition. They are not as brutal as original rules and use them alot more than I did the old Exhaustion rules.
-1 per level for all attacks/saves/checks/DC, -5 speed per level(min of 5ft speed) Additionally 1: 0 2: No Reactions 3: 1 Action or Bonus Action 4: Can't Ceoncentrate, Max 1 attack 5: Fail all Saves, Vunerable to all Damage
Two I really enjoy are Index Card RPG by Runehammer, d20 done nice and light.
The other is Breathless by Fari Games, it has a free SRD and tons of games for it released under the Breathless Jam https://itch.io/jam/breathless-jam/entries