While the results won’t be exactly the same, it is possible to get similar flavors using cucumber (deseeded first, then shredded) or jicama. However, if you can find semi-ripe papaya that is beginning to yellow but is not yet orange, this should also work.  Other versions of the dish use other ingredients like unripe mangoes, green apples, shredded cabbage, and even banana. However, I haven’t tried any of these.