lp-custom-login-message.php

<?php 

add_filter( 'leaky_paywall_subscribe_or_login_message', 'zeen101_custom_login_message', 10, 3 );

function zeen101_custom_login_message( $new_content, $message, $content ) {

	$zeen_message = '<div id="leaky_paywall_message">';
	$zeen_message .= '<h2>I want to use this message instead!</h2>';
	$zeen_message .= do_shortcode('[leaky_subscription_cards]'); // you can even add a shortcode or php function
	$zeen_message .= '</div>';

	return $zeen_message;

}