Magento – Tracking pixel on checkout success Magento 2

I'm trying to incorporate a tracking pixel from Share A Sale into my Magento 2 site.

Their instructions were as follows;

You will be marking changes to the following file:
/app/design/frontend/default/[templatename]/template/checkout/success.phtml

(if this file does not exist, please modify this file instead: app/design/frontend/base/default/template/checkout/success.phtml )

Find the line that reads:

<p><strong><?php echo $this->__('Thank you for your purchase!') ?></strong></p>
and just below that line, add the following code:

<?php
$orderId = $this->getOrderId();
$order = Mage::getModel('sales/order')->loadByIncrementId($orderId);
$total = $order->getGrandTotal();
$subtotal = $order->getSubtotal();
$discount = $order->getDiscountAmount();
$affiliateTotal = ($subtotal + $discount)
?>
<img src="https://shareasale.com/sale.cfm?tracking=<?php echo $orderId; ?&amount=<?php echo $affiliateTotal; ?>&transtype=sale&merchantID=66031" width="1" height="1">">"

However, the two files mentioned above dont' exist in Magento 2.
The closest thing I could find is in;
Public_html/vendor/magento/module-checkout/controller/Onepage/Success.php

In which the contents are;

<?php
/**
 *
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Checkout\Controller\Onepage;

class Success extends \Magento\Checkout\Controller\Onepage
{
    /**
     * Order success action
     *
     * @return \Magento\Framework\Controller\ResultInterface
     */
    public function execute()
    {   
        $session = $this->getOnepage()->getCheckout();
        if (!$this->_objectManager->get('Magento\Checkout\Model\Session\SuccessValidator')->isValid()) {
            return $this->resultRedirectFactory->create()->setPath('checkout/cart');
        }
        $session->clearQuote();
        //@todo: Refactor it to match CQRS  
        $resultPage = $this->resultPageFactory->create();
        $this->_eventManager->dispatch(
            'checkout_onepage_controller_success_action',
            ['order_ids' => [$session->getLastOrderId()]]
        );
        return $resultPage;
    }
}

I'm a front end developer so I possess a basic understanding of PHP that I use for creating themes, I attempted to incorporate the code but it breaks on $orderId = $this->getOrderId();

Any help or guidance would be greatly appreciated.

UPDATE:

I built the module and modified the files in the following manner per the first answer;

Success.phtml

<div class="checkout-success">
    <?php if ($block->getOrderId()):?>
        <?php if ($block->getCanViewOrder()) :?>
            <p><?php echo __('Your order number is: %1.', sprintf('<a href="%s" class="order-number"><strong>%s</strong></a>', $block->escapeHtml($block->getViewOrderUrl()), $block->escapeHtml($block->getOrderId()))) ?></p>
        <?php  else :?>
            <p><?php echo __('Your order # is: <span>%1</span>.', $block->escapeHtml($block->getOrderId())) ?></p>
        <?php endif;?>
            <p><?php /* @escapeNotVerified */ echo __('We\'ll email you an order confirmation with details and tracking info.') ?></p>
    <?php endif;?>

    <?php echo $block->getAdditionalInfoHtml() ?>

    <div class="actions-toolbar">
        <div class="primary">
            <a class="action primary continue" href="<?php /* @escapeNotVerified */ echo $block->getUrl() ?>"><span><?php /* @escapeNotVerified */ echo __('Continue Shopping') ?></span></a>
        </div>
    </div>
</div>
<?php
$orderId = $block->getRealOrderId();
$total = $block->getGrandTotal();
$subtotal = $block->getSubtotal();
$discount = $block->getDiscountAmount();
$affiliateTotal = ($subtotal + $discount)
?>
<img src="https://shareasale.com/sale.cfm?tracking=<?php echo $orderId; ?>&amount=<?php echo $affiliateTotal; ?>&transtype=sale&merchantID=66031" width="1" height="1">
<h1>Order Id: <?php echo $orderId; ?></h1>
<h1>Affiliate Total: <?php echo $affiliateTotal; ?></h1>

Registration.php

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'lfc_shareAsale',
    __DIR__
);

Module.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="lfc_shareAsale" setup_version="0.0.1" />
</config>

Success.php

<?php
namespace Vendor\Module\Block\Checkout;

class Success extends \Magento\Checkout\Block\Success
{
    /**
     * @return int
     */
    public function getGrandTotal()
    {
        /** @var \Magento\Sales\Model\Order $order */
        $order = $this->_orderFactory->create()->load($this->getLastOrderId());
        return $order->getGrandTotal();
    }
}

di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Checkout\Block\Success"
                type="Vendor\Module\Block\Checkout\Success"/>
</config>

File structure;

Commands ran successfully;

php magento module:enable lfc_shareAsale
php magento setup:upgrade

Issue now is that echoing $orderId and #affiliateTotal yield empty\0 values.