What can be used as a substitute for potassium nitrate as a preservative?
Nitrates and nitrites have very few practical substitutes. They work well as antimicrobial agents, have a not unpleasant taste, and are easy to work with. This is a surprisingly unique set of characteristics.

Using just salt as a preservative would work really well... as long as you are careful to keep the meat in a single piece, and are careful to cultivate the right bacteria. In the right conditions of temperature and humidity, these bacteria break down some of the meat into nitric oxide and, well, nitrates and nitrites. 

Celery juice works really well.. except that it's high in nitrates that then break down into nitrites in the curing process. It's also hard to predict how much nitrites will be released into the final product. 

If you are really worried about nitrates / nitrites, can safely omit them, but there are some important caveats. First, your antipasto will look... less cheerful. The hemoglobins in the meat will oxidize, and turn from red to grey without the nitrites. Nitrates and nitrites do create other flavors in meats besides just saltiness. You will also need to eat the antipasto fast- without the powers of preservatives, it'll spoil at the same rate as fresh meat. I would treat the final product as such, and would not leave it unrefrigerated for any amount of time

In short, without nitrates, you can make a delicious fresh sausage. Just please cook it, and don't call it a cured meat.