Welcome aboard! Good news - the KZ250 fuel cap was used on many KZ models. Below is a diagram with part number from Kawasaki.com and a list of different bikes the same cap fits.
Attachment KZ250FuelCap.jpg not found
KZ1000-A3 (KZ1000) (1979)
KZ1000-A3A (KZ1000) (1979)
KZ1000-A4 (KZ1000) (1980)
KZ1000-E1 (Shaft) (1979)
KZ1000-E2 (Shaft) (1980)
KZ1000-J1 (1981)
KZ1000-J2 (1982)
KZ1000-J3 (1983)
KZ1000-K1 (LTD) (1981)
KZ1000-K2 (LTD) (1982)
KZ1000-M1 (CSR) (1981)
KZ1000-M2 (CSR) (1982)
KZ1100-A1 (Shaft) (1981)
KZ1100-A2 (Shaft) (1982)
KZ1100-A3 (Shaft) (1983)
KZ1100-L1 (LTD Shaft) (1983)
KZ250-D1 (KZ250) (1980)
KZ250-D2 (CSR) (1981)
KZ250-L1 (CSR Belt) (1982)
KZ250-W1 (LTD Belt) (1983)
KZ305-A1 (CSR) (1981)
KZ305-A2 (CSR) (1982)
KZ305-B1 (CSR Belt) (1982)
KZ400-H1 (1979)
KZ440-A1 (LTD) (1980)
KZ440-A2 (LTD) (1981)
KZ440-A3 (LTD) (1982)
KZ440-A4 (LTD) (1983)
KZ440-D1 (LTD Belt) (1980)
KZ440-D2 (LTD Belt) (1981)
KZ440-D3 (LTD Belt) (1981)
KZ440-D4 (LTD Belt) (1982)
KZ440-D5 (LTD Belt) (1983)
KZ440-G1 (1982)
KZ550-A1 (1980)
KZ550-A2 (1981)
KZ550-A3 (1982)
KZ550-A4 (1983)
KZ550-C1 (LTD) (1980)
KZ550-C2 (LTD) (1981)
KZ550-C3 (LTD) (1982)
KZ550-C4 (LTD) (1983)
KZ550-M1 (LTD Shaft) (1983)
KZ650-H1 (CSR) (1981)
KZ650-H2 (CSR) (1982)
KZ650-H3 (CSR) (1983)
KZ750-E1 (1980)
KZ750-E2 (1981)
KZ750-E3 (1982)
KZ750-F1 (LTD Shaft) (1983)
KZ750-H1 (LTD) (1980)
KZ750-H2 (LTD) (1981)
KZ750-H3 (LTD) (1982)
KZ750-H4 (LTD) (1983)
KZ750-K1 (LTD Belt) (1983)
KZ750-M1 (CSR Twin) (1982)