Soft bags are pretty universal, but hard cases, not so much. The key is the mounting brackets: on soft bags, they typically include a zip on/off strap for under/over the seat mounting; on hard cases, they are metal brackets, which mate to your bike's frame. Sorry, but for a bike 30+ years old, you would likely need to troll e-Bay/Craigs List for Vetter luggage, made for your bike year and model -- they appear, but not too often. Shipping can get expensive, as well, as they are quite bulky.
Another option is to use detachable soft bags, and just carry them into your motel/hotel with you. That is what we did with our '79 K model. I bought a generic set of
metal guard brackets, to keep the bags out of the wheel's spokes, and to clip them to, to keep them from flapping in the wind, when empty (inexpensive carabiner clips through the fabric loops on the bottom of the bags, clipped to the metal brackets). I purchased
these brackets off of e-Bay; they bolted to existing bolts along the bike's seat (had a steel bracket for the hard trunk, which came with the bike). Take a look here, at the first & third photo's, to see them on my old bike:
link.
The included, wide, zip on/off 'strap' did not work very well for us. We made up some custom straps using webbing, and metal, duffel bag type, spring clips, from a fabric store, which went over the top of the bike's seat, to hold the bags up, above the exhaust. They were easy to make, had clips to quickly detach from the bags' loops on top (
most bags include a zipper on/off strap; if the over-the-seat strap, included with the bags, works for you, this won't be necessary...). Once you get a set of soft bags, you will see the loops, top and bottom, for attaching them. It only took a minute to attach/detach the bags; the bags came with carrying handles, so they were easy to use.
Otherwise, search e-Bay for
universal hard saddlebags, and see what you like/can use for your bike. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII