here's the mj50 petcock. It even has screws. I would just take the screws out, clean it and either cut a new gasket or maybe you'll get lucky and qt50 petcock gasket will work.
The pw50 petcock is the closest thing that I've seen that might work. You'd probably just have to take the two lines coming out of the tank and T or Y them into one line or just cap off one of the two lines and you'd lose your reserve function.
btw, are you buying petcocks from the idiot on eBay who seems to think that his qt50 petcock will work on the towny? I think his user name is custom-moto-pro out of Iowa. (edited)