You can, however, change them into a Kingdom and play as the Kingdom of Venice. If you don't want to mess with the scenario files, start a new game, save immediately, and open the save file.
Find the country block for Venice, will look like this:
country = {
tag = VENE
government = republican
etc
}
Change government to feudal instead, then add this block of text near the very top of the save file:
VENE = {
gender = male
type = duchy
dna = "82896235784151"
title = { C001 }
}
You will now be able to select Venice. It will show an inaccurate portrait and titles when loading the save game up, but that fixes itself automatically.