Are your battles random or evented?
If evented, set up duplicate monsters and duplicate troops, and change the drop items. Then use a conditional branch in the monster event to see if you have the quest, and call whatever troop has the appropriate monsters.
If they are random, you can set up the troop event to change the drop at the start of battle for those particular enemies, depending on whether or not you have the quest. I won't go into further detail until you state whether they're evented or random battles. If they're evented, you're good to go. If they're random, I'll walk you through the event commands you need to set up at the start of the battle. But I need you to give me this information first:
Set up your enemy to give the 'regular' drop item (when you don't have the quest).
Save any changes.
Open up the data/Enemies.json file in a text editor - Notepad++ is fine.
Locate the enemy and copy the whole line and paste it in here. It'll look something like this:
{"id":1,"actions":[{"conditionParam1":0,"conditionParam2":0,"conditionType":0,"rating":5,"skillId":1}],"battlerHue":0,"battlerName":"Bat","dropItems":[{"kind":1,"dataId":4,"denominator":2},{"dataId":1,"denominator":1,"kind":0},{"dataId":1,"denominator":1,"kind":0}],"exp":0,"traits":[{"code":22,"dataId":0,"value":0.95},{"code":22,"dataId":1,"value":0.05},{"code":31,"dataId":1,"value":0}],"gold":0,"name":"Bat","note":"max hp was 200","params":[2,0,30,30,30,30,30,30]},
And also let me know the item id of the kill point coin.
