You'll have to forgive me, as I've forgotten the exact scripting you'd need to input, but you can inherit properties/etc from other scripts.
It's essentially setting up a script like a property. So make a quest with a quest script, that can contain all of your variables and data, then inherit from that.
Let me look up one of my scripts so I can get you the exact line.
Edit: Got it-
ScriptName Property PropertyName Auto
So if your script was called MyScript, and you wanted to refer to it as... MainS, you would type:
MyScript Property MainS Auto
Then, to access it's properties, you just do MainS.Property. So lets say you make a property within that master script called MyInt, you could then manipulate that property, on the master script, with the following:
(MainS.MyInt) += 1
That would add 1 to the value of 'MyInt' in the master script.
Using this you can have one master script, and access it in any other script in the game.
I don't think I'm forgetting anything. I use this to count how many enemies are alive during certain quest events. I have a master script on the quest with a variable, and then in a script attached to the enemies, in an OnDeath() event I subtract 1 from the value of the master int, and if it's set to 0, I advance the quest. The result is that the player has killed all enemies, and the quest advances.