The simplest solution would be to install the workbench (the part of the AmigaOs not stored in the ROMs) from the Install disk. I've one sitting near my desk, I can send it to you as a lha compressed file, just send me a message.
You'll have to check that you still have all the wb disks:
Workbench, Extras, Locale, Storage and Fonts.
I'll give you instructions how to decompress the lha file on Amiga and put it on a disk, using CrossDos to read the files from a pc 720k disk.
Varthall