Theres seperate file for events. They tricker per turn and I'm not sure if its possible to change them per year. But if you dont mind typing a bit you could double every value there.

Another thing is that theres a game end turn. You need to find and edit that one too so the game doesnt end half way.

I dont have the expancion but here is where the file is in vanilla:
C:\Program Files\SEGA\Medieval II Total War\data\world\maps\campaign\imperial_campaign\descr_events.txt
Mayby someone can post the location for it in the expancion?