Is there any way to modify faction aggression AI?

9 times out of 10 (VI, Expert) AI-controlled Almos get pounded into dust. Same goes for the AI-controlled Byz.

Is there any setting that I can tweak to set all faction AI to the same (highest) level? In short, I want everyone to be as aggressive as, say, the Aragonese.

Go to campmap/startpos folder and open early/high/late/viking.txt, depends on your starting period.
Find section "Faction behaviour". Go to the line for the faction you want to change (Almos and Byzs). So, for example, change the line

SetBehaviour:: FN_ALMOHAD MUSLIM_PEACEFUL

into this

SetBehaviour:: FN_ALMOHAD MUSLIM_EXPANSIONIST.

As you can see there are many behaviours, so you can experiment with them a lot. But keep in mind that these are only behaviours set by default; faction behaviour also depends on situation in game. (I've seen Polish, set as defensive, becoming one of the major powers in the game)