At least that piece of plastic, which looks a lot like electrical conduit, is on the outside so it shouldn't contaminate the fuel inside the carburetor all that much. I had been guessing you'd put a soft plastic sleeve inside the thread, to make up for the missing part (I had visualised that most of the interior thread was missing). If I had been trying to fix the problem you actually had, I'd have filed or machined off the damaged bottom part of the carburetor body and used a short piece of aluminium tube (not copper against the aluminium carburetor - that would corrode) as a spacer to replace it, with a gasket on the upper end of the piece of tube. In theory that would be a permanent repair. You could always try that if your piece of conduit gives way after a few months - but getting a second-hand carburetor is better.
I wish we'd had those pictures to start with - we'd have had a different conversation.
