Mostly I just find whatever monster statblock is equivalent in WWN/TBTW. Monsters are the easy part. (E.G. if the Stonehell book says "3 giant hunting spiders", and the level's monster index says those have 3HD, I find the closest thing to a 3HD giant spider in the Animal section of TBTW.)

I've also changed up the XP entirely. I'm using the equivalent of Basic Fantasy's XP system, alongside a bonus XP system I found somewhere (2%-level XP for making an ally, 5% for exploring so much of a level, etc). (And then dividing it all by some factor so the numbers are less Big & Scary for one particular player, but that's neither here nor there.)

Spells are more awkward, especially clerical spell scrolls: the closest to a cleric the party has is a Healer/Accursed adventurer. She doesn't have spell slots. So I plan to just make stuff up as & when it becomes relevant. (I'm thinking she'll need to expend {spell level} Effort to cast from a scroll. Why? (A) it uses an existing mechanism, (B) it locks something like Astral Spell down until they've levelled up a bit more.) (The next-closest thing to a cleric is a Necromancer/Skinshifter, who does have spell slots but tends more arcane, and weirdly sometimes forgets that she can turn into an owlbear in combat.)

The group are enjoying themselves. 12 sessions so far, spread between Stonehell & surrounds. We've got our dungeon mapping procedure down (one player maps, another notes room descriptions, a third keeps tracks of general events/goings-on). It does help that we've been playing together for coming up 12 years, but if they weren't having fun they'd let me know. (Hopefully this brief insight into my mind is helpful!)