Others may have done similar, but I tried out advice I had received, converting a 7mm venturi 2 needle OS carb to 3mm by sleeving the barrel. I did this because I could not buy a sufficiently small venturi carburettor. I have tried making one but also I wanted to see if I could adapt a larger OS brand purchased one. The OS carb when stripped down has a cylindrical barrel with clearance holes for the jet needle and opposite that, the idle needle. A brass insert, in the original 7mm barrel cross drilling, machined to match the barrel cylindrical dimensions and drilled out to 3mm, with top and bottom drillings to remake the clearance for the two needles - was tried out.
Engine ran and it managed good throttle response. My conclusion is that such an adaptation is worth attempting and gives decent results.
I hope this may help or be of interest.