Have a good look, it might have fell down and be sat on top of the engine.
You can glue them back in, Ive had them come loose in other bikes especially using the cheap Chinese new inlet rubbers. They sometimes pull out when taking off the balancing pipes.
If it’s gone you could measure up another spigot and find a piece of new tube to glue in. Either superglue or high temp silicone works. Got to be preferable to taking the carbs off of everything else ok. You could even buy a spare rubber and do it next time you do carb cleaning.
