That's a simple O-ring. You've got to remove the governor "lid" to get at it though, remove the throttle cable to shaft linkage at the top, and then pull the throttle lever down through the bottom to get at the throttle shaft o-ring. Get a VE pump rebuild kit, they are cheap like under $20 and include all the crush washers, gaskets and o-rings for the pump.
Here you can see in this kit the throttle o-ring is number 4:
There is also a brass bushing. Those are not included with the VE pump rebuild kit, and would need to be pressed in and out. But if the throttle shaft doesn't have "play", just a leak, I'd try just replacing the o-ring.