Replace a single field with custom inspector

Is there a way to pick only one of the fields of my object in the inspector with custom code? Without having to replace everything else?

I want to replace the default int inspector with a list of EditorGUI.IntPopups instead of the default list of EditorGUI.IntField, without having to rewrite the whole thing

You should be able to create a custom PropertyAttribute and PropertyDrawer to do this. However, depending on what you are using this for, if you just want a specific list of ints and not allow anything else, you could probably just use an enum instead.