Magento 1 Order Email Template Preview

emailPreview.php

<?php 
require_once 'app/Mage.php';
Mage::app();
ini_set('display_errors', 1);
// loads the proper email template
$emailTemplate  = Mage::getModel('core/email_template')
    ->load(7);
// All variables your error log tells you that are missing can be placed like this:

$emailTemplateVars = array();
$emailTemplateVars['order'] = Mage::getModel('sales/order')->load(14513);
$order = $emailTemplateVars['order'];
$emailTemplateVars['usermessage'] = "blub";
$emailTemplateVars['store'] = Mage::app()->getStore();
$emailTemplateVars['sendername'] = 'noissue';
$emailTemplateVars['receivername'] = $order->getCustomerName();

$paymentBlock = Mage::helper('payment')->getInfoBlock($order->getPayment())
    ->setIsSecureMode(true);
$paymentBlock->getMethod()->setStore(Mage::app()->getStore()); 

$emailTemplateVars['payment_html'] = $paymentBlock->toHtml();

//displays the rendered email template
echo $emailTemplate->getProcessedTemplate($emailTemplateVars);

Add this file into magento root folder
Replace order id and email template id with your id