Because you said Briggs, gearing is your best choice.
Adjusting the governed speed on these, "if" you even have the complete governor is tedious, and requires pulling and reinstalling the shroud each time to check. Setting the governor is accomplished by setting the rod in different holes on the wind vane beneath the shroud.
You didn't specify which machine, but a 10t clutch going to a 72 tooth drive sprocket will get him slowed down. 17 MPH on a six-inch wheel based mini bike running at 3600 RPM. (Full throttle)