Running OSR modules with 5E: my 6 players are 60 hours / 5-6 levels into the Dwimmermount megadungeon using mostly-by-the-book 5e. Since we started with first level characters entering the first level of the mountain I weakened the kobolds & orcs
I use a lot of CR 1/4 Pig Orcs IMC:
Pig Orc GuardCR 1/4 (50 XP) PB +2
AC 14/16 (ringmail or scale, shield) HP 13 (2d8+4)
ST +2 DE +0 CO +2 IN -1 WI +0 CH -1
Battle Axe or Longsword ATT: +4 Damage: 1d8+2
Lance (long spear) (2h) ATT+4 Damage 1d12+2, reach 10', disadvantage within 5'
Javelin (4) ATT +4 Damage 1d6+2 range 30'/120'
Aggressive: As a Bonus Action, the orc can move up to its speed toward a Hostile creature that it can see.
I did do some CR 0 kobold mook stats but have not used them, I kinda like the very deadly 5e kobold.
Kobold MookCR 0 (10 XP) PB +2
AC 10/12 w shield HP 3 (1d6-1)
ST -3 DE +0 CO -1 IN -1 WI -2 CH -2
Dagger (1h) or kobold spear (1h): Melee Weapon Attack: +2, Hit: 2 (1d4) piercing damage. Spear does 1d6 2-handed.
Sling (2h loading): Ranged Weapon Attack: +2, range 30/120 ft., one target. Hit: 2 (1d4) bludgeoning damage.
Sunlight Sensitivity: While in sunlight, the kobold has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Pack Tactics: The kobold has advantage on an Attack roll against a creature if at least one of the kobold's allies is within 5 ft. of the creature and the ally isn't Incapacitated.