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.

You cannot substitute preservatives in recipe. Food safety is very hard to get right, sometimes small changes can have a very large effect.

If your recipe specifies saltpetre, you have to use saltpetre. Any change to the recipe, or using a subsitute, means that the result has to be tested in a laboratory before it can be declared safe.

If you cannot find any place to purchase more saltpetre, you could choose another antipasto recipe from a trusted source, looking for one which uses a different preservation method, for example canning. Note that you cannot can the original recipe and assume that it will be safe - not all recipes are suitable for canning.

Preservatives traditionally have been

Salt, Acid (vinegar, etc), Salt Peter (Potassium Nitrate, KNo3, never look up how this stuff is produced)

Vegetetable preservation usually relies on first 2, but meat usually involves all 3 for a real long term storage solution. You can use the first 2 to cure pork belly, but it won't last as long as bacon.

The safest way I've been told by a canner is to salt and acidify your preserves but really a pasteurization phase is the best you can do.