Hello,
The original System 7.1.x for the PowerBook 520 was never available for download from Apple. Were you thinking about the downloadable System 7.5.3 (nineteen files US), which is not a normal set of system floppies (once decoded, the files are segments of one large self-mounting disk image)?
To begin with, are you sure that the hard disk does not contain an operating system? Sometimes, a PRAM reset (holding down the four keys Command=Apple + Option + P + R until a second startup sound is heard) can help the computer locate a system folder. You may also want to try a Power Manager reset (PowerBook 100 through PowerBook 5300: Resetting Power Management Unit (PMU) - Apple Support).
A Network Access Disk 7.5 (http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/Utilities/Network_Access_Disk_7.5.sea.bin) will work as a boot disk for a PowerBook 520. In order to create a properly sector-copied floppy, you would need another old (pre-1998) Macintosh computer with a built-in floppy drive. Keep the downloaded file encoded .bin until on that machine. Once there, an appropriate version of StuffIt Expander is used to decode the .bin. Following that, Disk Copy 4.2 (http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/Utilities/Disk_Copy/Disk_Copy_4.2.sea.bin) allows you to create a bootable floppy from the Network Access.image (the Make A Copy button).
Post back if you do not have access to a suitable StuffIt Expander version, or if you do not have access to another old Mac. If absolutely necessary, there are ways of creating Mac floppies under certain Windows systems using special techniques.
Jan