Unfortunately I believe you are kind of stuck. The iBook if I recall uses a PRAM capacitor on the logicboard. That means a logicboard replacement or power supply replacement to fix matters. It is possible in replacing the hard drive, you accidently released some static electricity and damaged one of the components that helps it sleep.
You are right, it is a capacitor, they do not have a PRAM battery like other Macs.
Would it be possible to replace that capacitor with "normal/minimum" soldering skills? But you say, it could be I damaged something other on the logicboard, that is involved in the sleep process?
About the power supply: you mean the DC-in Board?
You think I really have to exchange the DC-IN-Board or could that http://www.fixya.com/support/t1660953-replace_clock_battery_ibook_g4_laptop as well do the trick?
Also found this helpful link http://www.thebookyard.com/product.php?products_id=1884
On ifixit, I got the usuall "did you try PMU reset? did you re-install the OS?" answers. The standard answer by Apple-Users "did you reset the PRAM/PMU", just like the standard Apple-tech-support answer way back in the days was "replace the Logic Board" (even if it was a faulty PSU in a MDD G4.)