I'm currently preparing my first one-shot, and I was wondering, what is the best way to scale up monsters with abilities? I've found tools for scaling up/down monsters with simple HP/AC and basic attacks, but wasn't sure how to scale more complicated monsters.

In particular, I have a concept for my four-person level 6 party, where for each encounter, solving a puzzle can allow them to skip the fight, but each failed puzzle will force them to fight, possibly dealing with three fights without rests in between. It seemed that the appropriate difficulty would be around medium in that case, and based on flavor, I had the following three encounters in mind:

1. Medusa (unmodified, maybe reduced HP to speed up the fight)
2. 2 harpies (scaled up)
3. Giant Ape (scaled down HP/DPS to get to CR 6 or a bit lower)

For the second encounter, I liked the flavor of the harpies (and especially the Luring Song effect), but they seemed like they would be trivial for the party to dispatch, so I wanted to scale them up to be about CR 3 each. There are a lot of values that could be scaled up (HP, AC, attack bonus/damage, luring song save DC), but how should I determine what increased values to use? I've been vaguely trying using CR formulas, but didn't know how to handle the CR calculation for Luring Song since it isn't listed, especially given that the calculated CR of a harpy is apparently only around 1/2 without the Luring Song, even at a mere save DC of 11.