The original system for a PowerBook 2400c/180 was 7.6.1. Other supported systems (according to Apple's technical specifications) were 8, 8.1, 8.5, 8.6 and 9. You would need to purchase an appropriate Apple system CD. Ensure that it is a retail disc (for all models), not a disc for a certain (other) machine.
You can use one of the Apple CD-ROM drives above, providing that you have the proper HDI-30 cable. See the manual for details about connecting SCSI devices and about booting from a CD.
If necessary, and if the already installed operating system is working (and you can navigate the Japanese menus), you should at least be able to create a Disk Tools PPC floppy on the PowerBook. Use the Disk Copy 6.3.3 program (the Make a Floppy command under the Utilities menu) for this. I assume that you have the external floppy disk drive for the PowerBook.
www.info.apple.com/support/oldersoftwarelist.html
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/System/Mac_OS_8.1_Update/Disk_Tools_PPC.img.bin
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/Utilities/Disk_Copy/Disk_Copy_6.3.3.smi.bin
One may also want to try to modify a Disk Tools PPC to hold CD-ROM software, thus allowing a CD to be read when the computer is running from the floppy disk system.
Additionally, since the computer in question has PC Card slots, an inexpensive PC Card adapter for CompactFlash and a CompactFlash memory card can be very useful for file transfers. In certain PowerBook models, such a CompactFlash card (reformatted to Mac) can even act as a startup disk (if it has an appropriate system folder).
Jan