Remember that First Edition tends to throw huge numbers of low-level opponents at a party. Flipping through my copy of the original, in the outdoor areas we have, among other encounters, a group of 70 goblins and another group of 81 hobgoblins.
This module also marks the first appearance of a number of creatures and magic items, including Xag-ya and Xeg-yi, Behirs, Slippers of Spider Climbing, and the Horn of Fog. If the PCs have encountered these before, consider replacing them with new, "unknown" monsters and magic items.