Quickest and cheapest would be to pick up a brake switch for the rear master cylinder, (they are fairly generic) and whip up a simple wiring harness. The harness will contain a resistor that reduces the power to the bulb until the brake switch bypasses it and send full power to the bulb. For a horn, a bicycle bulb horn will suffice, I believe. A friend of mine dual-sported his pumkin this way and I have the brake switch and a wiring harness that he made me so that I can do my KDX.
A lot of it depends on your local police station that will do the inspection and sign off as it being street legal. Our Secretary of State here in Michigan has a form with a checklist for the police to inspect. Some items are more strictly enforced than others, and as I said, it really depends on the officer doing the inspecting. A friend, relative or bike friendly cop is your best bet. I believe that Baja Designs makes a full dual-sport kit for most bikes. I would check them out.
Good luck and let us know your results.