I ended up renovating a brand new Chinese motorcycle. Its chief complaint was a broken fuel tank. The petcock mounts to a three-inch stalk of polyethylene that is plastic welded to the main tank, and the plastic weld failed after 240 miles.
I looked around on the Internet to find a replacement tank, but parts are not so easy to find for a Chinese motorcycle, especially when you don't know the trick. So then I researched adhesives to bond the stalk back on, but they were almost as expensive as a plastic welder, which was way more money than a plastic fuel tank should cost.
Then I learned the trick of Chinese motorcycles. Many of them are the same or close enough to have interchangeable parts, even though they're made by different manufacturers. That makes it easier to find parts for them. I was used to thinking that any given model of motorcycle would be made by only one manufacturer. Like the way cars are. Only the manufacturer Toyota makes the model Camry. Only Mazda makes the model RX-8.
Chinese motorcycles aren't like that, though. There are a whole bunch of manufacturers for a particular model. For instance, what I was working with was a HiSun 200GY. If you go to look for parts for a HiSun 200GY, good luck. You won't find them. But the model 200GY is actually made by a ton of different manufacturers, so if you widen your search for just 200GY parts, then parts are available, though not conveniently.
I eventually found a fuel tank that has a long built-in metal petcock stalk that bolts directly to the tank instead of having a plastic stalk at all, and even though it was for a bike made by a different manufacturer, it pretty much almost fit with minimal effort.
Figure 1: This is the gas tank. It has been taken off of the bike. The right side of this photo would be toward the rear of the bike as assembled, the left side toward the front. The top of this photo is the top of the gas tank. So we are looking at the left side of the tank as it would be mounted.
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Figure 2: Notice that the petcock is mounted to an approximately three-inch black polyethylene stalk that is plastic welded to the polyethylene tank itself. This tank failed at that plastic weld.
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Figure 3: You can see the stalk separating from the tank.
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Figure 4: You can see one of the fuel filters poking up out of the stalk.
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Figure 5: This joint was made with polyethylene plastic welding rod. There are only a couple of adhesives available for bonding polyethylene. One of them is made by 3M. It is very expensive but it seems like the one that would be most likely to work on this joint.
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