Did you know you can change the model used in a particular car slot by just changing the default.ide file in the data folder, instead of replacing any of the models? Open up the file with a text editor, find the Kaufman's line (number 216), and change the first two words of the line to 'zebra', just like the Zebra Cab's line (number 188). Save the file and load the game, and it will use the zebra.dff and zebra.txd files for the Kaufman Cab.
Alternatively, in the same file, change the word 'ignore' in the Zebra Cab's line to 'taxi', and it will make it spawn on the roads alongside the Kaufman Cab.