What are you trying to do specifically: keep the dead longer, of keep the dead permanently?
Delete this line of code for each unit's proto entry to keep the dead permanently:
<Flag>CorpseDecays</Flag>
I don't know how to change the time it takes for a corpse to decay.
Mummy zombies on the Nile map is actually more historically accurate than tanks being destroyed by pikemen.
Want denser forests and a
huge Amazon?