Modify Posts from Custom_Post_Type within the plugin

Use the_content filter to add your fields to the output of the_content() function:

function wpd_content_filter( $content ) {
    if ( 'your_custom_type' == get_post_type() ){
        if( $meta = get_post_meta( get_the_ID(), 'foo_meta', true ) ) {
           $content = $content . $meta;
        }
    }
    return $content;
}
add_filter( 'the_content', 'wpd_content_filter' );