...separate valve would be over engineered, and theres probably not enough room to do it between the tank and carbs/airbox. I had a leaky one on a 78F tank. You can try putting a set of vice grips on each of the pins and give it a good sinch down. This my tighten things up enough to stop the leak. Whey can't you drill the pins out? You should be able to drill them out and tap the holes for screws. While u'r at it post over on "parts needed". Someone may have a good one they are willing to let go. Be sure its a 77/78 petcock...the bungs are different on earlier years...
