Try adding this:
'order' => 'DESC',
'orderby' => 'meta_value',
Or if you don’t plan on adding more meta_query params, this format may work too:
$replaceme = get_posts([
'posts_per_page' => -1,
'post_type' => 'custom_post_type',
'meta_key' => 'custom_field_name',
'meta_compare' => '>=', // optional
'meta_value' => 'custom_field_value',
]);
I also don’t know the exact syntax but just play with it until it works. 🙂