Here's a little information for you. The typical fuel tank petcock is not a pipe thread. That is, it is not a tapered thread but a metric machine thread which relies on a gasket to seal. I've done several tanks this way and never had a leak: Use a 1/8" pipe tap to rethread the tank then you can easily locate a petcock at almost any hardware store. I run the tap in carefully and not too deep. It will rethread the tank so it will accept any NPT (National Pipe Thread) 1/8" pipe fitting. Use teflon tape or a thread sealer compatible with gasoline to assure a leak proof installation. Good luck.
Tom