I did some research and the only poisonous snake in Greece which is also conveniently a viper is called the Ohia snake (Vipera ammodytes). Although when comparing that species to the 2 most famous statues of medusa's head (sculptors: Bernini and Cellini) the snakes do not match the Ohia. The head shape is similar but the scales on the heads of the snakes in both works are completely different from the head scales of the Ohia. They are more similar to the leopard snake or the European cat snake, neither of which are venomous. Another thing to consider is that one statue portrays the bodies of the snakes as completely smooth (Bernini) and the other as textured (Cellini). The textured snakes imply that the snake used to inspire the art would have had keeled scales which the Ohia snake does have and both the leopard and cat snake do not. One last important detail is that the Ohia snake has a distinctive horn that protrudes from its nose which is not depicted in either art piece. But it is worth noting that not every Ohia's horn is as prominent. Sorry for the nerd rant and I've attached pictures of each species and sculpture below.
European cat snake photo European cat snake:
Ohia snake photo Leopard snake:
Cellini's Medusa photo Ohia snake:
Bernini's Medusa:
Cellini's Medusa:
