add_action( 'init', 'json_init' );

function json_init() {
    add_feed( 'json', 'viper_json' );

    add_rewrite_tag( '%json_id%', '([0-9]+)' );

    add_rewrite_rule( '^feed/json/id/([0-9]+)/?', 'index.php?feed=json&json_id=$matches[1]', 'top' );
}

function viper_json() {
    if ( ! get_query_var( 'json_id' ) )
        exit( 'missing id' );

    $id = get_query_var( 'json_id' );

    $post = get_post( $id );

    echo json_encode( $post );

    exit();
}