I believe that in larger cities, a Hunger Artist could gather a crowd simply by advertising it as performance art or conceptual art. For example, such a thing wouldn't seem out of place in large, art-centric cities that already host a great deal of performance art. In these cities, people would probably go with their friends to watch the Hunger Artist, eat food in the room if they were allowed, and then talk about the impact it had on them and their own food choices. While most people would skip the exhibit in favor of other options, enough people would see it that ticket sales would probably stay relatively healthy. I could even see the morning breakfast being a big draw for crowds of people.
In smaller towns, I think it would be harder for people to sell tickets to something like a Hunger Artist after the initial fervor died down. Such a strange exhibition would be more unusual there and probably met with more suspicion if the Artist himself wasn't a member of the community. They might also be more open to recognizing how unhealthy such a thing is for the performer himself since there are less extreme performance art pieces in smaller places.
Personally, I wouldn't attend something like that unless the proceeds were being used to help feed people who go hungry because they don't have enough food. There are enough hungry people in the world who don't have a choice; it doesn't feel right to go see someone who is simply choosing to starve to make money.
Seeing such a performance would also not be very satisfying because not much would happen. From Kafka's description, it's simply a person fasting watched over by guardians who ensure that they don't eat. That isn't much to look at.