Welcome to the joys of never being able to find parts. Particularly tyres. Looks pretty clean though, so you'll save a bit in not having to replace everything.
Also, I can't remember if these things have a siamesed intake port, so if you ever want to replace the carb, you can just find a single barrel job from something else and adapt that. If they still have individual intake ports, then find a single 40mm sidedraught.