I'd try what you've got 1st. I've never needed to replace one, just clean them out and lube them so they turn easy and smoothly. There's a rubber disc with 4 or 5 holes in it behind the lever. The lever base seals against this but often if the holes in the back of the lever are sharp edged, they'll tear up the rubber disc. I "break" the edges of any sharp-edged holes on the lever using a small sharp pocket knife and if the rubber disc is torn up, I flip it over. The backside is usually pristine and will seal up fine again without having to replace the rubber disc.
I don't disturb the backside of the petcock where the vacuum diaphragm is unless the petcock still leaks after doing the front side. The diaphragm is kinda delicate. There is a small plunger with an o-ring on it attached to the diaphragm and this fits into a beveled hole in the petcock body. The vacuum pulses work this plunger in and out of the beveled hole to start and stop fuel flow. Sometimes the o-ring is worn out and needs replacement or the beveled hole in the body is dirty or corroded so needs a cleaning.