Syslog is missing a lot of time, can't see the rebuild, disk21 needs a filesystem check, you can try that before using the old disk, but without seeing what happened with the rebuild probably best to use the old disk as there might be data corruption.
You can't do that without doing a new config, if you just want to check the data in the old disk you can do that with UD, make sure the array is stopped first, you can then do the new config and re-sync parity with the old disk.