Translation function to modify the thank you page text. Place this function within your themes functions.php file or a site specific plugin.

function.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function mycustom_filter_gettext( $translated, $original, $domain ) {
 
    // This is an array of original strings
    // and what they should be replaced with
    $strings = array(
        '%1$sCongratulations%2$sYour registration has been successfully processed.%3$sCheck your email for your registration confirmation or click the button below to view / download / print a full description of your purchases and registration information.' => 'Thank you for submitting payment. Your order has been successfully completed. It is important for you to complete the TEAM REGISTRATION form if you are playing in the croquet tournament. Click the button below to visit the registration form.',
        // Add some more strings here
    );
 
    // See if the current string is in the $strings array
    // If so, replace its translation
    if ( isset( $strings[$original] ) ) {
        // This accomplishes the same thing as __()
        // but without running it through the filter again
        $translations = get_translations_for_domain( $domain );
        $translated = $translations->translate( $strings[$original] );
    }
 
    return $translated;
}
 
add_filter( 'gettext', 'mycustom_filter_gettext', 10, 3 );