When I first saw an illustration of a peryton: half-eagle, half-deer, I found this very inspiring. But the actual monster stats aren't that interesting. It has 4 HD, an AC 7, a fly speed about twice as fast as a human, it is immune to normal weapons and gets +2 to hit due to sharp antlers. I also remember reading that they hate elves and harvest hearts from a freshly killed body. Strangely, their claws are sharp enough to tear a heart out and even carry some aloft, but too weak for fighting? Seriously?
Here is my take on the peryton: let's keep the HD, 4 is a nice challenge. AC 7 would equate to AC 13 in a straight conversion and proly AC 15 in Lotfp. Let's say they are tough and their AC equates to chainmail (AC 16). Then, since they are flying birds and have big flapping wings that will mostly likely disorient the combatants, I would give them AC 17. Tough critter. Since magic weapons aren't common in Lotfp, I would do away with the resistance to ordinary weapons. Rather, I would do it backwards: the peryton is vulnerable to magic. Magic attacks do twice their hit points in damage. That's also why they hate elves. Elves have magic. The get an attack bonus for sharp claws. That's also rather contra-productive in Lotfp, where magic armor is also rare. So rather than an attack bonus, let's have them do extra damage. They'll use their sharp claws, because using the antlers for anything more than mating games is impractical. The Monstrouos Manual gives their damage as 4-16 (4d4 I guess). A bit much... I'd scale that down to 2d4+2 giving an average of 7 damage and making them quite lethal.
The end result is a weaker peryton useable at lower levels, but one I feel will still be a challenge to the adventuring group.
Peryton: HD 4, AC 17, Speed: flighing twice as fast as a human, on the ground half as fast as humans, Morale: 9, Damage: 2d4+2, Vulnerable to magic (twice damage), AL: Chaotic
Tactics and special abilities: they will always attack elves first, because they just hate them. A peryton will not change the target of its attack, until either the target or itself is dead. As soon as they have slain an opponent, they will tear his heart out (obstacle to raise dead...), this will take one round during which they are vulnerable to attacks (AC reduced to 14). When they have the heart, they will end the attack and fly away.