A question that comes to my mind liverwort, having replaced nipples on cap locks often myself is, what is the size of the original nipple that you are replacing? If you can determine that you could set about finding a replacement that would not necessitate drilling and re-tapping threads in the drum.

Or, is that actually the case here, that you are replacing a CVA nipple with the same size and you just want to get a tap to clean up the threads before installing the new one?