Yes, it's in campmap\startpos\[era].txt file, just add this line after these with starting data:

SetGameEndDate:: 1700

ie:

SetStartDate:: 1087
SetPeriod:: EARLY
SetEarlyPeriodStartDate:: 1087
SetHighPeriodStartDate:: 1205
SetLatePeriodStartDate:: 1321
SetGameEndDate:: 1700

---

Flaming ammo... yes...

In file PROJECTILESTATS.TXT there are projectiles named:

flaming_arrow
flaming_rock
flaming_trebuchet
flaming_mangonel

Try to copy their stats replacing these existing in already used projectiles. Or just note the last collumns:

FlamingProjectile IsFlaming

And change their values. For example use longbow as flaming, change longbowmen to normal bowmen etc.

Sadly, I don't know codes for flaming ammo (as are for short bow = sbow, crossbow = xbow etc.).

For easier editing use great Dragon MTW editor:

https://forums.totalwar.org/vb/showthread.php?t=33392