No idea if it's original, but I'd guess yes. Did you try cleaning the gasket & fuel tank mating surface? If it still isn't sealing, maybe the gasket has hardened. If hard, you could try some of the methods for softening rubber, but they usually don't last. Liquid Wrench silicone spray lubricant is a good cleaner for rubber & plastic. I use it to spot clean just about everything on the motorcycle.
Replacement. From the diagrams at Partzilla the 1980 KZ440-B1 had the same fuel cap as the B2 plus some of the earlier KZ400 & KZ650 models. The other USA 440s had the newer style rectangular fuel cap.
Search EB or post want ads on the various forums. or try a motorcycle salvage yard. In the mean time I would clean & lubricate it and check to see if a new gasket is available. Z-power UK lists it.