We’ve been using 1E monsters in the game we call “Advanced Freebooters”.
The extra HP that characters earn as they level up makes it feel fair and fun to give the monsters some number of d8 Hit Dice, plus damage rolls on multiple dice (count armor against each attack).
We’re also converting armor class to armor which gives some monsters a LOT of armor, but with high level FotF character levels, it’s really been working out.
It does make the game more crunchy, but crunch can be very near and dear to us old school gamers.
My vision is to create a new “Manual of Monsters” also detailing instinct and special moves for our 1E favorites.