I found a kit on eBay:
https://www.ebay.com/itm/226233476688
It might be discontinued again, but there's a zillion of them out there. With Honda's parts, whenever the last 3 digits become -670 (or nearly anything "6xx") it indicates that Honda has stopped stocking it (again) and it has been turned out to the vendors who made it for them. This is when we can often find them "out there", just not at Honda's dealers.
I think this petcock kit has been on-off for years. 
Thanks for the info Mark, but that's the wrong part.
Hmm..those are the only seals in the petcock: the rest of the body is a valve. If the valve itself is leaking, the whole petcock must be replaced, which must be where you started?
The Zamac body and spigot (the part with the lever on it) was cast and made as one unit. Then the other seals were added, going up to the tank. MTBE ate at the Zamac during those years when it was in our gasolines, making the surfaces irregular and not smooth and straight. Then, yeah, they weep fuel afterwards.
Then you ARE stuck with finding a new petcock.
Our modern fuels are not a lot better: this is why I advocate for adding some oil to the fuel, which helps a lot to neutralize the non-gasoline effects on the metals. Just one example from Denver, CO: there is butane (or was, from October 1 until March 31 and then however long it takes to deplete the gas station's tanks) in our fuel now, and it dries out and cracks any rubber line in the fuel system. Then, as soon as it moves for any reason, it weeps gas, with the usual results, fires, etc. So, I have ceased riding my own 750 between those dates, as I got tired of replacing the fuel hoses every stinking year because of it. Oil in the gas greatly reduces the effect, but it still removes the flexibility from the hose itself in just one season, turning them into a soft plastic that breaks when flexed 180 degrees.
