According to the question and the answer here, it is quite unresolved if Animagi can or cannot perform magic. However, is it possible to perform magical spells on Animagi (spells such as Stupify, Rictesumpra or any other common spells)? Except the spell to reverse transform animagi back to their human form, are there evidences of other spells cast on animagi?
Remus Lupin and Sirius Black threatened to perform magic on Wormtail should he transform. By definition, their threat would only have been carried out if Wormtail was in his rat form.
"I'm going to tie him up," said Lupin. "That's all, I swear."
Harry stepped out of the way. Thin cords shot from Lupin's wand this time, and next moment, Pettigrew was wriggling on the floor, bound and gagged.
"But if you transform, Peter, growled Black, his own wand pointing at Pettigrew, too, "we will kill you. You agree, Harry?"
Harry looked down at the pitiful figure on the floor, and nodded so that Pettigrew could see him.
(Prisoner of Azkaban, Chapter 19, The Servant of Lord Voldemort).
"One wrong move, Peter," said Lupin threateningly, ahead. His wand was still pointed sideways at Pettigrew's chest.
(Prisoner of Azkaban, Chapter 20, The Dementor's Kiss).
It's unclear exactly what spell they were proposing to use on Wormtail but it's a fair guess that it was Avada Kedavra. Since both Lupin and Black were deadly serious in their intent to revenge the Potters it's very unlikely that they would make this sort of threat if they were not capable of executing it.
We don't know whether other forms of magic work when cast upon Animagi. As far as I'm aware, this is the only instance of magic being performed on, or threatened towards, an Animagi. (Hermione performing an Unbreakable Charm on the jar containing Rita Skeeter is the next-closest example, but it scarcely counts). I don't see any reason why spells shouldn't work as they would upon normal animals since the Animagi becomes the animal in almost every respect.