Do you still have the installer for GarageBand 4.1.2? It might work, if the installer is not an installer for a PowerPC version of GarageBand, but an Intel version.
Another thing you could try would be to migrate the missing items from the instrument library and Loop library.
You will need to copy all missing items from these folders:
/Library/Application Support/GarageBand/Instrument Library/
(look for missing instruments in all subfolders )