Questions abt MCStacker Mob Creation

So I've been trying to use MCStacker for a while now to make mobs and I have some questions 1. How do you fully replace a mobs head for an item? 2. What's the attribute that controls if a mob burns in the daylight? 3. Will MCStacker just automatically fill in variables that I don't myself? (I fell like this somewhat obvious but I just wanna make sure) 4. More a question for just general datapack stuff, but, how would I go about putting the mobs I make into a full on datapack?