Not so good!
It doesn't look like you're going to have much luck with downloading that rom. Maybe someone else has the files you need and can send them to you. I wouldn't hold my breath, though, because Proto is long disappeared, and so the rom is not being developed or supported.
You don't mention if you have tried the watapon rom. Have you read this thread:
https://www.oesf.org/forums/index.php?showtopic=6304&hl=There seems to be a fair amount of activity around it. I would suggest that since that rom is being actively developed, and like the tkc rom is sharp based, maybe that might be an option for you?
I assume you didn't know that you can take a nand backup? You could have taken a backup of your whole system before you started experimenting.
You don't say which processor type you have, PXA250 or PXA255. Take a look at this page:
http://www.galacticslacker.com/zaurus_sl5600.shtmlThere is a nand backup file there which is based on tkcROM. You need to write to the owner and ask for the link. You could use that to get back to where you were, so long as you have the correct processor, read carefully and heed the warnings!
Good luck,
Mike.