If you're looking to replace your worn out petcock and don't mind losing the vacuum operated on/off, an Acerbis universal-style petcock will fit the Seca II tank. The o-ring that Acerbis stocks with the petcock is a bit small so to run the Acerbis petcock, you might need to make a paper gasket. It's cheap, comes with reserve and on/off. You'll want the Acerbis right side petcock (p/n 2044230998).. It will match up the same as the stock Seca II petcock did.
Note: The one shown below is the left-side petcock (for my BMW tank). You want to buy the right hand petcock.