To me, a "good case" (meaning decent shape and sufficient protection) is more important than "original case" (which wouldn't really be original in your case, since it didn't come with the guitar in question). But, to give you an idea of what to look out for, I have owned/seen a couple of "Guild" cases from the late '70s early '80s...
Over the years, I have owned a couple (four I think) archtop dread cases from that period with the gold script "Guild" logo that had rather spartan purple felt interiors. Very sturdy, but not a lot of padding inside... I made do by padding with T-shirts.
My '76 D-50 came to me with its "original" first owner case: an un-branded (meaning no Guild logo) arch-topped 5-latch dread case with (rather bright) blue plush interior. I just recently found and bought another exactly like it (but a bit poorer cosmetic condition) as a "70-80s Guild case" on ebay (for my D-46). With shipping, it cost ~$100.
There have been several "Guild" cases on eBay recently that seem to be leftovers/seconds from the Tacoma factory. I got one for ~$70 plus shippin... a TKL archtop dread case with a brass oval "Guild" plaque near the handle. Quality is pretty good, and it was a good fit for my dreads. I think it only has 3 latches plus a hasp, tho'.
My favorite case is a near-mint (when I found it) top-of-the-line TKL with double neck support and crushed velvet interior that fits my dreads like a glove... which I bought used at a local shop for $65!
Good hunting!