Find a silencer for your golf cart. Try your golf cart dealer, but don't be surprised if your golf cart manufacturer does not make one. Maybe your dealer will have a universal silencer available.
Try ATV, or all-terrain vehicle, dealers. Many silencers are available for ATVs, both model specific and universal. Whether your engine is a four-stroke or two-stroke, you should be able to find at least a universal silencer that fits your engine and cart.
Install the silencer according to the manufacturer's instructions.