This sounds like a lot of fun! I’m interested in what you mean by “old-school 5th Edition.” Just running OSR-style with the Basic Rules?
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The book is intended to be workably system-neutral, and to not require outside statistics or tables for resolution.
Creatures, spells, items, and the like do have their own chapters of statistics blocks appropriate for the fifth edition of the big game - Challenge Ratings, GP costs, etc.
All sections rely on some classic OSR mechanics - d6 tables, hexagonal travel areas, growth/evolution via hit die (HD), and a focus on wildlife and wild environments.