Is this a real Deere ZTR, or a Home Despot Deere ZTR? I suspect there is a big difference in alternator capacity between the two.
Does Deere have a kit for your mower? If so, check to see if it uses halogen bulbs or 1156 tail light bulbs.
How many amps does your alternator put out? Two 55-watt halogen lights require 9.6-amps.
I would suggest checking Peterson and a number of other manufactures for tractor work lights. There are plenty out there. I got my Peterson's from Northern Tool. If you put two on, I suggest one trapezoid and one flood to get near and far coverage.