Originally Posted by 2door
Check with the vendor you bought the tank from first of all.
Externally threaded petcocks have been used for years on motorcycles. You might want to check a local Japanese motorcycle shop if the seller of your tank can't help you.
If you're fortunate enough to have a motorcycle salvage yard in your town you might find one to fit there.
Typically those types of valves (petcocks) have a reserve position with two inlets in the tank.
If all else fails and the bung isn't internally threaded you could run a 1/8" pipe tap into it and use a standard NPT petcock or use a 10mmX1 tap and use a kit petcock with a gasket.
Hmm, thanks for the tips. I could borrow my friends tap set so I might give that a try.