With the nut on the fuel bowl ,you can't really use a rattle gun or use force as you know the whole lot will spin and as you say it's difficult holding the inside part from spinning.
Looks like it's tight on the last of the thread when removing ,I would get some Loctite and put it on the thread of the 2 brass parts that came apart then tighten the 2 parts and leave it for 24 hours to harden ,then put some vice grips on the upper end then tighten the nut at the lower end ,use penetrene on the thread ,keep working the thread by tightening and loosening, eventually as you loosen the nut should undo a mm more then tighten again and repeat many many many times eventually the nut will come off but you don't use a lot of force as this can damage or seize or break the brass thread .
So you're just working the thread backwards and forwards until eventually it will undo more and more.This can take a while.
With the 2 compensating tubes (no. 16) that screw into the emulsion tube ,they are just two threaded bolts with a hole in the middle so could be made ,just have to drill the right size hole.
The Victa carby sounds a little bad to me on this Pope,I can hear the engine popping in the video all the time , when the carby fuel mixtures are wrong you get that popping noise out the exhaust, even in my video I hear the popping noise when decelerating but not when throttling up and revving.
Here is a bike making the noise when decelerating but the Pope video you hear it all the time popping ,I wonder if too small a carby is working like a bigger carby with the choke stuck on.The right carby may also help that smoke problem.
