I spent a bit of time working on this and just finished a Midnight Campaign with my group (none of which had played Ravenloft before) and we found that using 5-7 players starting at L7, this campaign was doable, but difficult.
Mostly, all I did was convert the monsters over from what they were back when I6 came out, to something more standard for 4E. This could easily replace Kalton Manor on the Nentir Vale map if you are running that sort of campaign.
I made this encounter run specifically so all you need is the Dungeon Master's Book, Player's Handbook I, AD&D I6 Ravenloft, and Monster Manual I. The only real issue I had was I had to take out the Rust Monster and replaced it for a Troll... it didn't fit very well so I added in the alternative, which would require the Monster Manual II.
All NPCs should be played as Human Rabble (Monster Manual I pg.162) unless otherwise noted.
All Zombies will be played as L2 Zombies (Monster Manual I pg.274) unless otherwise noted.
All Wolves should be played as Worg (Monster Manual I pg.265) and their numbers divided by 2 unless otherwise noted.
All Gypsies should be played as Human Bandits (Monster Manual I pg.162) unless otherwise noted.
All Ghosts should be played as Phantom Warriors (Monster Manual I pg.116) unless otherwise noted.
All Ghouls should be played as Ghouls (Monster Manual I pg.118) unless otherwise noted.
All Wights should be played as Deathlock Wights (Monster Manual I pg.262) unless otherwise noted.
All Wraiths should be played as Wraiths (Monster Manual I pg.266) unless otherwise noted.
All Vampires should be played as Vampire Spawn Fleshripper (Monster Manual I pg.259) but with (6d10+20) HP unless otherwise noted.
All Gargoyles should be played as Gargoyles (Monster Manual I pg.115) and should appear in numbers no greater than (1d4) unless otherwise noted.
All Groaning Spirits should be played as Wailing Ghost (Banshee) Monster Manual I pg.117) unless otherwise noted.
All Rust Monsters should be replaced with Spirit Devourers (Monster Manual I pg.68) unless otherwise noted. An alternative to this is to use Rust Monster (Monster Manual II pg.178) and multiply the number by 1.5 (rounded down)
E1. Bildrath’s Mercantile
Parriwimple should be played as a Human Berserker (Monster Manual I pg.163) but with (5d10+60) HP
Replace the four Red Dragons to just two. They are played as Adult Black Dragons (Monster Manual I pg.75)
K8. Great Entry
Replace the eight Gargoyles with just four Gargoyles (Monster Manual I pg.115)
K28. King’s Worship Place
The Strahd Zombies here are replaced by two Mummy Guardians (Monster Manual I pg.192)
K32. Maid’s Hall
Helga, vampire still functions as normal as outlined, but while attacking she plays as though she were a Vampire Lord (Monster Manual I pg.258)
K35. Facing Guardians
Replace the 2 Wraiths with Four Wraiths (Monster Manual I pg.266)
Replace the five Giant Spiders with ten Deathjump Spiders (Monster Manual I pg.246)
KM. Familiar Room
For the three Black Cats, when players advance on them they will dart back into the shadows and disappear. They cannot be caught.
The Witches are replaced by two Harpies and five Bloodfire Harpies (Monster Manual I pg.154)
K69. Guard’s Quarters
The ten Skeletons are now twelve Skeletons (Monster Manual I pg.234) And there is a 1d4 chance that there will also be a Skeletal Tomb Guardian (Monster Manual I pg.235)
K72. Office of Vengeance
Replace the Shadow Demon for a Dark Stalker (Monster Manual I pg.49)
K75. South Dungeon
The Warewolf has statistics from the Lycanthrope Warewolf (Monster Manual I pg.181)
K78. Brazier Room
The two iron golems are replaced with two Flesh Golems (Monster Manual I pg.142)
K84. The Catacombs
Replace the one Ghost with one Phantom Warrior (Monster Manual I pg.116)
Replace the one Specter with one Specter (Monster Manual I pg. 244)
Replace fifteen Wights with fifteen Wights (Monster Manual I pg.262)
Replace one Vampire with one Vampire Spawn Bloodhunter (Monster Manual I pg.259)
Replace one Banshee with one Wailing Ghost (Banshee) Monster Manual I pg.117)
Replace three Huge Spiders with three Deathjump Spiders (Monster Manual I pg.246)
Play the Trapper as a Zombie Hulk (Monster Manual I pg.275)
Replace the three Hellhounds with three Hell Hounds (Monster Manual I pg.160)
Replace one Nightmare with one Nightmare (Monster Manual I pg.196)
Count Strahd von Zarovich
Create a Level 14 Vampire Player Character that will represent your usage of Count Strahd von Zarovich. Create it using the strongest tactics in mind with clever usage of skills, feats, and ability modifiers as well as magic equipment. If you are unable to do so, you may use the Vampire Lord (Monster Manual I pg.258) with an extra 6d10 HP and all of the abilities and spells listed in his description (I6 Ravenloft pg.3)