Install a factory Deere muffler, but the important part is that you add enough pipe on top of the muffler to get the sound above the opeators station.
I will surely agree that those spark ignition engines are loud. I have a friend with a 3020 gas with a huge chromed straight pipe. He brought it to me to figure out why is was down on power. I had to run it on the dyno several times to get all the kinks worked out. Under 100% load the neighbors a mile away could hear it.