We're assuming you have a tank that comes with the Chinese 2 stroke engine kits. The threads are 10mm X 1. It is a machine thread. But, a 1/8" pipe thread will fit although you might have to use some thread sealing product. 1/8" pipe thread valves are available from many sources. Grainger, Tractor Supply, and even some hardware stores.
Increasing the flow through the petcock isn't necessary. If the valve flows a steady stream that will be sufficient unless you have a highly modified engine and run WOT, wide open throttle, all the time. It is a myth that increasing fuel flow from the tank will increase performance.
Look at it this way. If you open the petcock and it flows a steady stream, measure how long it takes to drain your fuel tank. Now put everything back together and go ride your bike. Ride it until the tank is empty. Even if you run WOT the whole time, I guarantee that you'll run longer, much longer, that it took to drain the tank through the petcock.
The engine can only consume so much fuel. The float valve in the carburetor controls how much fuel the engine needs and delivers it. Increasing the flow through the petcock will not increase how much fuel the engine can use.
The kit petcocks can cause problems. They can get restricted which is usually because the in-tank filter gets clogged from trash in the tank. That's why we stress cleaning the tank before installing it. The valve can also malfunction where it will not completely stop the flow of fuel or restrict it. However, if when open it flows a steady stream and shuts the flow off when closed, there's nothing wrong with it.
Tom
