The power-level and tone is just too dissimilar. To give you an idea of what I would do to adapt it to 5E:
Races allowed: humans.
New races: Dolmenwood elf, moss dwarf, grimalkin (tabaxi), woodgrue (goblin).
Classes allowed: Cleric, Fighter, Wizard, Thief, Bard, Paladin, Ranger.
Then I would...