In , Hilchos Shechitah » Chapter 3, Halacha 18, the Rambam writes:
Whenever we have used the term "unacceptable," the animal is a nevelah and if a person partakes of an olive-sized portion of it, he is liable for lashes for partaking of a nevelah. For only an acceptable slaughter as commanded by Moses our teacher of blessed memory prevents an animal from being considered a nevelah as we explained.
In Sefer Taharah » She'ar Avot haTum'ah » Chapter 2, Halacha 1, The Rambam writes:
When a non-kosher domesticated animal or wild animal was slaughtered, it does not impart the impurity of a carcass as long as it is in its death-throes unless its head is cut off. Instead, it is considered as impure food.
Similarly, when a kosher animal is slaughtered in an unacceptable manner and it is still making convulsive movements or one of its signs or the majority of one was slit, it is not considered impure at all until it dies.
Halacha 7 says:
When an animal that was tereifah was slaughtered in a proper manner, even though it is forbidden to be eaten, it is pure.
It would seem that as long as the shochet didn't touch it after it stopped convulsing, he would not become impure. If the shechita itself was proper, and the animal was later found defective, it would not be (or impart) impurity at all.