The Dungeon Master's Guide to Cynidicea offers a fair bit of expanded information that would help develop this module into a campaign.
Dragon #315 has an article and map (for version 3.5) expanding the module entitled "Mystara: Return to the Lost City"(pp. 88-92). The article is difficult to utilize alongside the module, however, because in the article Zargon has a Challenge Rating of 16 and the difference between D&D (Basic) and version 3.5 is significant.
Dungeon #142 has an adventure (for version 3.5) entitled "Masque of Dreams" (pp. 14-30) that once again pits the adventurers against the Zargon cult.
Classic Modules Today B4: The Lost City
is designed to assist in converting the module for use with D&D 5e
Note that the first iteration of PDF scans sold by Wizards of the Coast omitted the 4-page pullout black & white map section (Tier 6, Tier 7, Tier 8, Tier 9, Tier 10, Elevation, Underground City Map) that was included in the print module. Later PDF iterations included these missing maps. These interior maps should not be confused with the blue & white maps on the module's cover/folder interior (Tier 1, Tier 2, Tier 3, Tier 4, Tier 5, key).