You may find that the vacum seal (the black things either side of the yellow square) is torn or doubled over allowing fuel to seep through.
When looking for a Gpz part also remember to look for ZX600 as well as this is what they were sold as in USA. An '86 model is a ZX600A2 in USA speak.
This petcock is common to KZ550/Z550, KZ750/Z750, ZN700 from 1982 to 1987 and the GPz600 for 1985 - 1987. Part number for the full unit is 51023-1186. Also search by the part number as you may find that as well.
You should be able to find the part from any of the old Kawasaki parts sellers in the USA (ZX600 A1 or ZX600 A2 is the model). Just found one for US$83.00. When a rebuild kit is about $30.00 - $40.00 not bad value for OEM unit.
Good hunting.