Your best bet would be to cut the tube off at the base and get another threaded tube re-welded on. If its imperial tube then you may find it difficult, as thanks to being in the EU you can only buy French (metric) tube now unless you can find an engineer with imperial tubing, or he can roll some metal up to the desired size but it would be a fiddely job. Have the threads completely gone? You maybe able to get a dye run down them to restore the thread. I thnk there was a post on here a while back offering a really good metal working service, good luck