Sunless Citadel and Forge of Fury (hailed as modules that can easily be dropped anywhere but when it comes down to it the ways people integrate them into settings are seem pretty finagled because of the Ashardalon connection. It's not as easy as it seems to drop these in without much effort).
Crucible of Freya and Tomb of Abysthor. I am thinking Frey can be Mielikki (I used Elhonna in Greyhawk). Unsure whether Orcus cultists should stay Orcus cultists or change to another deity.
I've adapted Forge of Fury for AD&D 2E, and I'll be running it to my players in the near future.
In my own FR campaign, I've chosen the Forlorn Hills in the Sword Coast as Khundrukar's location, because I think it ties nicely with the history of that region.
Regarding Orcus, you may keep it around, because at least one FR sourcebook (FR9 - The Bloodstone Lands) mentions it. Or you could use Myrkul instead, depending on the time period.
Orcus plays a huge role in the Bloodstone Wars, to the point that the Heroes of Bloodstone Pass eventually go to Orcus's home plane and straight-up kill him (he gets better). I think there's pretty widespread fanon that this is the reason Orcus doesn't mess around much with Toril after that. But there's no reason his cultists can't still be around.