I have no rules citations for this, and won't attempt to present it as part of the handbooks, because it's not. It's the great part about being the DM: creative, realistic storytelling.
###The Situation
I count a few dead bodies, the monsters that killed them, the monster's and character's loot, and some scary rumors of evil things lurking in the forest. Enter, the new PCs.
###The Story
As DM, you want your world to be logically complete. That is, things that happened should make sense, past events should impact future ones, etc. When the new PCs arrive in town with their extra level, perhaps they hear about the "foolhardy, impetuous, and hubristic hooligans who got slaughtered by their lack of preparation." Maybe the guards are rebuilding the town's defenses to be ready for future harrassment.
Your players remember what they fought, but the characters don't (and shouldn't). If they want to get an idea of the beasts, they should have to ask around or track it, and we all know how good commoners are at exaggerating stories.
So, now what? The PCs might be able to prepare better for their encounter with bloodthirsty whatcha-ma-callits, but the monsters will also be ready for them. Change up their tactics to make the fight a little more difficult, but remember that it was a TPK the first time around. Perhaps the PCs can recruit a few guards to come with them.
Finally, as a reward (assuming they defeat the little beasties), they can scavenge the monster's hoard, finding the corpses' gear and whatever treasure the monster's had lying around.
Sounds like a nice, story-driven, non-immersion-breaking way to (a) have another stab at the same (but different) encounter, (b) retrieve old loot, and (c) keep the world consistent.
###To Directly Address the Question
No, there aren't many guidelines other than "you're the DM; it's up to you." This is but one possibility out of many, and actually presumes that they start with normal starting equipment, with the chance to get more gear before and after the battle. Another alternative would be to steal the corpses' gear while the monsters are roaming about, avoiding the fight altogether. It's your group's story. Have fun with it.