EX model engine 0.020 lower assembly questions

I see two issues:

The needle valve is leaking you need to add some fuel tube to help seal the threads

Add a nylon washer to help make a seal. I fabricated one using the lid material from a coffee can.
That thickness is suitable. You can use some 000 size steel wool and clean up that corroded venturi.

As an alternative:
You have a carb body design that requires modification, flat file that arch down level.
I bought two like that and it's the best solution. You will have to remove the carb body
and rinse it out well after you are satisfied of a flat surface.

I seal everything with after run oil when assembling to help reduce air leaks and
for fighting off corrosion / helpful when you take it apart later down the road.

The carb body from EX model is not made correctly and it has been discussed elsewhere on this forum or possibly on RCgroups.

You need to flatten the sealing surface or simply get a proper carb body from Coxinternational instead: https://coxengines.ca/cox-.020-carburetor-body-red.html