wecr_simple_product_message.php

<?php
/**
 * Allows you to change the default message for simple products.
 * 
 * @param string  $message      stock Easy Content Restriction message
 * @param int     $product_id   The post_id for the product the message would be tied to, may be a product variation id
 * @return string               Your custom message
 */
function  change_simple_product_message( $message, $product_id ) {
  return 'Our custom message';
} 
add_filter( 'wecr_simple_product_message', 'change_simple_product_message' );