Add custom fields capability to JSON-feed Wordpress plugin

json-feed.php

<?php
/*
Plugin Name: JSON feed
Plugin URI: http://wordpress.org/extend/plugins/json-feed/
Description: Provides feeds in JSON form
Version: 1.3
Author: Chris Northwood (modified by Dan Phiffer)
Author URI: http://www.pling.org.uk/
Contributors: @jimakker
Comments: works on WP 3.4.1
*/

add_filter('query_vars', 'json_feed_queryvars');

function json_feed_queryvars($qvars)
{
  $qvars[] = 'jsonp';
  $qvars[] = 'custom';
  $qvars[] = 'date_format';
  $qvars[] = 'remove_uncategorized';
  return $qvars;
}

function json_feed()
{
    $output = array();
    while (have_posts())
    {
        the_post();
        $output[] = array
        (
            'id' => (int) get_the_ID(),
            'permalink' => get_permalink(),
            'title' => get_the_title(),
            'content' => get_the_content(),
            'excerpt' => get_the_excerpt(),
            'date' => get_the_time(json_feed_date_format()),
            'categories' => json_feed_categories(),
            'tags' => json_feed_tags(),
	    'custom' => json_feed_custom_vars(get_post_custom())
        );
    }
    if (get_query_var('jsonp') == '')
    {
        header('Content-Type: application/json; charset=' . get_option('blog_charset'), true);
        echo json_encode($output);
    }
    else
    {
        header('Content-Type: application/javascript; charset=' . get_option('blog_charset'), true);
        echo get_query_var('jsonp') . '(' . json_encode($output) . ')';
    }
}

function json_feed_date_format()
{
  if (get_query_var('date_format'))
  {
      return get_query_var('date_format');
  }
  else
  {
      return 'F j, Y H:i';
  }
}  


function json_feed_categories()
{
    $categories = get_the_category();
    if (is_array($categories))
    {
        $categories = array_values($categories);
        if (get_query_var('remove_uncategorized'))
        {
            $categories = array_filter($categories, 'json_feed_remove_uncategorized');
        }
        return array_map('json_feed_format_category', $categories);
    }
    else
    {
        return array();
    }
}

function json_feed_remove_uncategorized($category)
{
    if ($category->cat_ID == 1 && $category->slug == 'uncategorized')
    {
        return false;
    }
    else
    {
        return true;
    }
}

function json_feed_format_category($category)
{
    return array
    (
        'id' => (int) $category->cat_ID,
        'title' => $category->cat_name,
        'slug' => $category->slug
    );
}

function json_feed_tags()
{
    $tags = get_the_tags();
    if (is_array($tags))
    {
        $tags = array_values($tags);
        return array_map('json_feed_format_tag', $tags);
    }
    else
    {
        return array();
    }
}

function json_feed_format_tag($tag)
{
    return array
    (
        'id' => (int) $tag->term_id,
        'title' => $tag->name,
        'slug' => $tag->slug
    );
}

// Return custom values
// author: @jimakker
// usage:
//	if paramater "all" is passed will show all custom fields (for security concerns it's better if you don't use this unless you're debugging, and remember to remove/comment those lines once in production)
//	else, will show the custom field named as the parameter (only one for now), returning null for inexistent value
//	if no parameters are passed will return "null"
function json_feed_custom_vars($custom_vars)
{
	if (get_query_var('custom')==''){
		return 'null';	
	} 
	// Delete these following lines if you don't want to expose all your custom fields
	else if(get_query_var('custom')=='all'){
		return $custom_vars;
	}
	// This is OK, do not delete
	else {
		return $custom_vars[get_query_var('custom')];
	}
}

add_action('do_feed_json', 'json_feed');

?>