That is it! I am about to open a window! I have had it with...ah, ninja'd.
One thing you can do is reskin existing monsters, particularly those with a poisonous theme (turn all the spiders into snakes), or add a snake theme to existing monsters. (This will also save you time on statblock conversion.) Maybe a rust monster has snakeheads in addition to rusty tentacles, a beholder has ten snakes on its head each with a different breath weapon and a giant snake mouth instead of a central eye that eats magic, the otyugh is actually a filth snake that has vestigial arms instead of tentacles, and of course the dragons could take after snakes more than lizards as usual.
A plague of supernatural winged, metallic serpent constructs is invading a small demiplane reknown for its lush greenery- its idyllic environs making it a place valued to beings of all kinds as a sacred neutral ground. The snakes are ruining all of that, and various factions are accusing each other of being behind the incursion.
The party is approached by a frustrated Demigod, Dubh Patrick, who is has recently been appointed guardian of this revered land. He explains, he has had it with the metal feathered snakes on his mighty floral plane. He wants the party to help him drive them out.