I'm sure somewhere out there, someone has written rules for using demons in old school D&D. This is my simple(?) take.
Basic Demon Template:
Demons are immune to sleep and charm spells. NOT hold spells. Silver weapons can cause either half damage (lesser demons) or 1 point of damage (greater demons) on a successful hit. Magic weapons cause full damage. Demons can be turned as undead 2HD stronger than their listed HD. i.e. a 7 HD demon is turned as a 9 HD undead. Demons will often have resistance to certain damage types, usually fire (auto half damage, save for none).
Demonic creatures are unnaturally strong and will gain an additional +1 damage modifier to any physical attack
As for the monsters themselves, stick this template on any other monster, and you've got a demon.
Additional Generic Demonic Powers. Different demons will have different powers.
Force Grab - Ranged attack w/in 30'. Save vs Paralysis or be thrown 2d6' taking 1d6 damage. If thrown against a wall, the demon may maintain the force, pinning the target in place.
Hellfire - Ranged Attack, 30/60/90 causing 1d6+1 damage per 3 HD of the demon
Corrupt - Melee attack, drains 1d3/1d4/1d6 points of Charisma. Damage can be restored with 1 week/point of damage or Greater Restoration (or better) magic.
Unholy Aura - Any lawful or neutral creatures within the aura must save vs spells or suffer a -2 penalty to attacks, AC, and Saving Throws while within the aura and for an additional 1d6 rounds. Aura size varies based on HD of demon.