You have to change the exhaust manifold and buy the down pipe. If you are handy you can reuse your vertical muffler and add a rear bracket under the axle.
I always run the horizontal style as I would tear a vertical one off in short order under the limbs and branches where I mow.
When I converted mine I went to an electrical supply outfit and bought a weather tight knockout cover to plug the hole in the hood and keep water out.
Painted it Blue. It looked nice.
Click on the link below. They sell both the manifold and exhaust pipe here on this site.