I use the 3e version of Thracia. My conversion to 5e system:
AC: Halve AC over 10.
Hit Points: Increase by 50%.
Attributes: Halve attributes over 20 (monster) or cap at 20 (NPC)
Damage: Increase by 50%.
Attack Bonus (weapon or spell) & Spell Save DC: Calculate based on CR-derived Proficiency + Attribute bonus
Good Saves: If it's a Good (2+1/2 level) save in 3e, the monster has Proficiency in it in 5e.
Will > WIS and possibly INT or CHA. Ref > DEX. Fort > CON and possibly STR.
This is easy enough to do that I can convert during play. I rarely worry about the exact number of spell slots an npc caster has, they will be dead in 2-3 rounds anyway...