Fans are pretty standard 40mm x 25mm. There are small rubber grommets that screw in to drop them into the fan mounts, but the screws are pretty standard fan screws. So yes - you can replace them.
Do be careful though - 1U takes a good deal of airflow to keep things cool and happy. You typical low-speed Noctua or other "silent" fans are likely not up to the task (if you can even find them in 40x25 flavors).
Do be careful though - 1U takes a good deal of airflow to keep things cool and happy. You typical low-speed Noctua or other "silent" fans are likely not up to the task (if you can even find them in 40x25 flavors).