If you can't find aji amarillo, you can make a paste from some fire-roasted and peeled orange or red bell peppers, with red Fresno chilies for some heat. While this does get you close, you should definitely try to find a jar of the real stuff, since it does have such a unique flavor.
You can also use russet potatoes.
You can use fresh peas that have been blanched or frozen peas that have been thawed.
Substitute red onion for the shallot if desired.