if the petcock is bad, there are 2 options, and the one to use is determined by how its failed..
1: Leaky petcock. you can rebuild them for cheap, or if you dont want to rebuild, you can use the following option as well...
2: physically broken/damaged. replacement. either with the factory type, or with a TW200 petcock, which bolts directly on to the tank. The TW200 petcock does not have the vacuum port, so it allows fuel to flow as long as its in the 'on' or' reserve' position. Also, it has only 1 outlet fitting, you would need to use a T fitting to hook the 2 fuel lines the vulcan has to the one port. many people have done this mod with no problems. The TW200 in a LOT cheaper than the original part, and remember to try to avoid the cheaper chinese knockoffs, as a few have had issue with them specifically.