In general, most monsters have inflated in hit points, chance to hit, and average damage per round across the editions in a steady number inflation. There are exceptions though, as every edition has monkeyed with the balance of monsters (often unnecessarily IMO) so that for example giants and dragons were tougher in 2e than 1e, but arcanodaemons were weaker. Demons were much tougher in 3e than they were in prior editions with balrogs in particular getting a huge boost, while stock humanoids like gnolls or lizardmen were relatively weaker (something reversed by 5e's bounded accuracy).
There is one big change in the game that is going to complicate this and that is that in AD&D 10th level was considered the "top level" and monsters level - the predecessor of today's "Challenge Rating" - went from 1 to 10 (or rather I to X, using roman numerals). So today's CR 20 monster has no real good counterpart since monster level X was the point that things topped out at. You can come up with a different scale to extend monster level upwards (and I have and have used it in places) to get a better feel for what challenge a monster presents to characters above 10th level, but that's a house rule and not RAW.
Still, I think you are pretty solid for both your choices. I don't have my books on hand, but I think Mummy was Monster Level V, and so to drop to CR 3 means that it is in fact relatively weaker. I don't remember Will O' Wisps monster level, but it was a viable end game monster that you could challenge high level characters with so it had to be around Monster Level VIII. To drop to CR 2 represents a massive loss of threat.
Other choices that I think are viable: Shambling Mound, Roper, and Black Pudding. These monsters have all lost relative threat and some special attacks and defenses that made them especially challenging in 1e AD&D. Shambling Mound is no longer resistant to basically all damage, no longer has a death attack, and it's attacks still do basically the same damage even though PC's have vastly more hit points. Roper has lost several special defenses and its attacks still do basically the same damage even though PC's have vastly more hit points. The black pudding is fairly close to its old nasty 1e AD&D self and still eating equipment (which let's face it is one of the scariest things a creature can do), but manages to be a mere CR 4 monster - which says a lot about how 4d8 damage and 10HD has become relatively less scary than it was in 1e AD&D.
UPDATE: The more I think about this, the weirder it gets. Shadow, always a nasty monster if you didn't have a cleric around to turn them, is still basically its old nasty self. It's strength drain was slightly nerfed from 1d6 to 1d4, but if anything it actually hits harder than it used to because it also does significant Necrotic damage. It's otherwise mostly unchanged, and yet the new version which on paper might be worse is only CR 1/2. What does CR even mean in this edition? The Black Pudding is CR4 yet the Purple Worm is a robust CR 15.
I'll also echo the person who said Spectre by noting just how badly nerfed what was once one of the most feared creatures in the game is. This creature is now CR 1. Not only has it lost the real bite of level drain, but for some reason they reduced it's HD. So an encounter with say 6 of these which once would have been terrifying in pretty much any edition is now pretty blasé once you hit the mid-levels.