I do have the patcock described in original post as manual alternative (linked on eBay). It wouldn't fit GZ without some modifications. The knob in reserve position will interfere with the tank weld seam. There might be similar petcock with distance from knob to mounting flange larger that is more suitable for GZ as non vacuum operated replacement.
I purchased this one. Not sure if the screws are same distance but gave it shot. Will let you know if it works. It looks like valve it further from mounting flange:
http://www.ebay.com/itm/321479366875...%3AMEBIDX%3AIT