There isn't problem like that in 1648 - it runs only for 30years (12 turns per year). In other cases you will have to a monitor checking if that character exists and spawning a new one if required. Which is rather tedious if you have many...
Code:
monitor_event FactionTurnStart FactionIsLocal
if not I_CharacterExists [name1]
spawn_character [faction_name], [name1], ....
end_if
if not I_CharacterExists [name2]
spawn_character [faction_name], [name2], ....
end_if
end_monitor
This way you will only have one single monitor which shouldn't slow down the processing speed.