The five factions playable is the easy part, there´s some file where the playable and unlockable factions are stored, I´m not sure which it is, but there is one.
The timeframe can be done rather easily as well, there is a start and end date somewhere in the campaign-related files.
Now the hard part, the leaders. You can name the initial members of your faction, those you start the game with, but not their decendants, unless you restrict the list of names to the few you want, but then you´ll get quite a bunch of Hannibals. You also can set the initial diplomatic relations, but they may and most likely will change in time.
You definitely can´t edit the winning conditions. It´s fifty provinces plus Rome, period.

I hope that is helpful in any way.