WordPress 3.5: custom gallery broken after upgrade

Yes, I read slevin’s post but I’m having a hard time making heads or tails of it, so…

EDITED & UPDATED BELOW

I think I’m closer but I’m not sure…now the page returns all of the images contained within the entire media gallery, versus only the ones attached to this specific post’s gallery. It’s formatting it correctly, with the larger image above the content and the thumbnails below the content and everything in the correct wrapper, but there are only 3 images attached to that post.

Page url: http://www.vintagepowertrain.com/lkjdkfljdklfj/

Here’s the code I have now:

function wpo_get_images(
$size = 'thumbnail', 
$limit = '0', 
$offset = '0', 
$big = 'large', 
$post_id = '$post->ID', 
$link = '1', 
$img_class = 'attachment-image', 
$wrapper = 'div', 
$wrapper_class = 'attachment-image-wrapper') {
global $post;

$images = get_children( array(
'post_parent' => $post->id, 
'post_status' => 'inherit', 
'post_type' => 'attachment', 
'post_mime_type' => 'image', 
'order' => '', 
'orderby' => 'menu_order ID') 
);

if ($images) {

    $num_of_images = count($images);

    if ($offset > 0) : $start = $offset--; else : $start = 0; endif;
    if ($limit > 0) : $stop = $limit+$start; else : $stop = $num_of_images; endif;

    $i = 0;
    preg_match('/[gallery.*ids=.(.*).]/', $post_content, $ids);
    $attachment_ids = explode(",", $ids[1]);
    foreach ($images as $attachment_id => $image) {
        if ($start <= $i and $i < $stop) {
        $img_title = $image->post_title;   // title.
        $img_description = $image->post_content; // description.
        $img_caption = $image->post_excerpt; // caption.
        //$img_page = get_permalink($image->ID); // The link to the attachment page.
        $img_alt = get_post_meta($attachment_id, '_wp_attachment_image_alt', true);
        if ($img_alt == '') {
        $img_alt = $img_title;
        }
            if ($big == 'large') {
            $big_array = image_downsize( $image->ID, $big );
            $img_url = $big_array[0]; // large.
            } else {
            $img_url = wp_get_attachment_url($image->ID); // url of the full size image.
            }

        // FIXED to account for non-existant thumb sizes.
        $preview_array = image_downsize( $image->ID, $size );
        if ($preview_array[3] != 'true') {
        $preview_array = image_downsize( $image->ID, 'thumbnail' );
        $img_preview = $preview_array[0]; // thumbnail or medium image to use for preview.
        $img_width = $preview_array[1];
        $img_height = $preview_array[2];
        } else {
        $img_preview = $preview_array[0]; // thumbnail or medium image to use for preview.
        $img_width = $preview_array[1];
        $img_height = $preview_array[2];
        }
        // End FIXED to account for non-existant thumb sizes.

        ///////////////////////////////////////////////////////////
        // This is where you'd create your custom image/link/whatever tag using the variables above.
        // This is an example of a basic image tag using this method.
        ?>
        <?php if ($wrapper != '0') : ?>
        <<?php echo $wrapper; ?> class="<?php echo $wrapper_class; ?>">
        <?php endif; ?>
        <?php if ($link == '1') : ?>
        <a href="<?php echo $img_url; ?>" title="<?php echo $img_title; ?>">
        <?php endif; ?>
        <img class="<?php echo $img_class; ?>" src="<?php echo $img_preview; ?>" alt="<?php echo $img_alt; ?>" title="<?php echo $img_title; ?>" />
        <?php if ($link == '1') : ?>
        </a>
        <?php endif; ?>
        <?php if ($img_caption != '') : ?>
        <div class="attachment-caption"><?php echo $img_caption; ?></div>
        <?php endif; ?>
        <?php if ($img_description != '') : ?>
        <div class="attachment-description"><?php echo $img_description; ?></div>
        <?php endif; ?>
        <?php if ($wrapper != '0') : ?>
        </<?php echo $wrapper; ?>>
        <?php endif; ?>
        <?php
        // End custom image tag. Do not edit below here.
        ///////////////////////////////////////////////////////////

        }
        $i++;
    }

}
}