Is it worth upgrading and repair

Sadly not ;-{

This system uses a PowerPC CPU and Apple has moved onto the Intel CPU's.

Apple has also long since dropped support in the MacOS (OS-X) for the older PowerPC systems, in addition, Apps written for the newer OS’s won’t run on your older PowerPC and using this system online would not be smart as the OS has a lot of security holes.

If you plan of using this system for simple writing with the supplied apps that it came with and don’t connect it to the internet in any way it could still be useful.

As for upgrading not much you can do here. Review this systems specs iBook G4/800 12”