Again, there's no easy way to do that.
You might be familiar with this assumption and I can tell you where it steams off too. D&D 4th edition is quite a different game with some similarities that were kept to brand it as "D&D".
The main problem here is that monsters only have powers they can actually use during an encounter. No more 1st level spells on your epic dragon. No more "the monster has this spells provided it has the time to buff". Everything in 4e happens during combat. And 4e Monsters don't have to deal with save-or-die spells, while most solos have to deal with multiple stuns (which didn't happen in 3.x).
AoO work in a different way, ranged attacks deal damage based on Dex, spells deal bonus damage based not on the level but on the mental scores. 5-feet step is now a movement action (except for kobolds)... Different game.
The easiest thing you can do is to take existing D&D 3.5 monsters or NPCs and present them to the group as if they were the kind of interesting monsters you want to portray.
Maybe choose something that has similar mechanics if you can find it.
In other word(s), reflavor.