OK, most are push fit but I have seen some with a screw in fitting. You could try using something like ‘liquid metal’ or ‘Devcon’ to secure it back in or look online for an 1/8th BSP to 6/7mm fitting like https://tinyurl.com/yyaozty6 and carefully adapt the carb top to take it.
In the meantime I have a spare carb top (copy not original Solex) to get you back on the road whilst you sort yours out if you e mail me at: osl282.info @ gmail.com
David Faulkner