I tried to install it yesterday but discovered that the bung and plug are now rusted/welded together - there was no getting the plug out.

        In a previous incarnation as a foreign car mechanic, I have had similar problems getting out broken spark plugs, CV joint bolts, exhaust manifold studs, etc. Here are the most effective procedures:

        Heat. It may seem counter-intuitive, but heating the plug intensely will loosen it. Logic tells you it will expand and get tighter, but years of experience have showed me that heating the plug works every time. I have heard lots of good guesses, but no one has come up with a definitive explanation.

        If you haven't rounded the head to nothing, and you can still get a good solid twist on the stuck plug, heat the crap out of it with an acetylene torch, then it will come right out. If you can get an impact gun on it, have the gun all ready to go beforehand, so you can loosen the plug while it is still red hot. A propane torch "kind-of" works, but does not have nearly the success rate of an acetylene torch.

        If you have broken the plug or rounded it hopelessly, there is still hope. Find a strong bolt that is close to the diameter of the plug, perhaps a tad smaller.  (I like to use VW Bug lug bolts.) Get the impact gun ready. Weld the bolt to the stuck plug and then hit it with the impact gun quickly, before it has any chance to cool. This works best with a stick welder, but MIG works too if you turn the current up and make a wide heavy weld.

        Of course, ground the welder to the pipe as close as possible to the plug and disconnect $en$itive electronics.