I just came to write the same thing. I don't remember ever seeing "viscera is" or "viscera was". Google gives 57,600 hits for "viscera are" and 46,900 for "viscera were". Out of the first ten hits for the former, nine are direct, ordinary uses of the word (the tenth is "CINEMA VISCERA are proud to present"), and it's ten out of ten for the latter.

For "viscera is", Google gives 45,300 hits--but out of the first several dozen I looked at, only one was a direct, ordinary use ("By this way, the viscera is not easy to be demaged [sic]"). Other uses include "Viscera is a band", "Perotin, c.1200: Beata viscera is part of these releases", "Situs inversus of the abdominal viscera is not a rare condition", "'Viscera' is choreographed", "Disposal of viscera is a problem", "Viscera is a non profit organization", "Viscera is a great word to know", etc. "Viscera was" gives 18,200 hits, which are of the same sort as the hits for "viscera is", such as "Viscera was released in the summer of 2000".