I do not know how 3.5 works but I do know that for old school it is pretty easy.
Figure that the HQ dungeons are perfect for 5-10th level dungeons.
Goblins are 1 HD monsters with a D4 damage attack. Save as 0 level men at arms.
Orks are 2 HD monsters with a D6 damage attack. Save as a 1-5th level fighter.
Fimir are 3-5 HD monsters depending on where they are placed in the maze, D8 damage attack. Save as a Fighter 1-5. Could just substitute Lizard Men and get away with it well enough.
Zombies 1 HD monsters with a D8 attack. Immune to any spell that may affect mind. (they are mindless...) Save as a fighter 1-5.
Skeletons. 2 HD monster with a D8 attack. Imune to mind effecting spells. Save as a fighter 1-5.
(most of those listed monsters you could just use by the book as listed in the sourse manuals and consider the Fimir a Lizard Man.)
Mummies in HQ are no where near as powerful as D&D mummies. So we have to through away the book here or consider that HQ mummies are just more powerful Zombies. Give them 3HD and a D10 attack. Save as a fighter level 6+.
The
Blood Thirster (gargoyle) could be an issue. The D and D gargoyle could do the job but it isn't really a gargoyle and we all know that. It is a Blood Thirster and the closest thing in D and D for that is some form of Demon or Devil like the Pit Fiend. But this monster is way out of class with the rest of the monsters....
Chaos Warriors are easy enough. Figure they are the equivilant of a level 6 Fighter with full Plate and a dex bonus of +2. Weapon damage is D10 +2.
Chaos Wizzards....ok this is can be an issue. I would go with a simple level 5/5 Magic Usser/Cleric. The witch lord I would make a level 10/8 Magic Usser/Cleric.
Spells...to much of a pain in the ass to figure them all out. Just go with the D and D spell rules for the casters.
So it is said that if you know your enemies and know yourself, you can win a hundred battles without a single loss.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.
~Sun Tsu The art of War~