Never seen a tank with only one vent. They all had two. Two styles were out there, one with a fill and overflow on the tank itself, the other had two vents on opposite sides of the needle valve.  These were difficult to fill as the fuel vents were very small and 1/8" fuel hose just won't fit them. Best tactic is to get a syringe with a 18 gage needle from a feed store and use that.

I'll put this here to start.
https://www.coxengineforum.com/

There are many styles of back plates over the years, and several different fuel fill arrangements.

The one-nipple back plate has a "pinhole" adjacent to the fill nipple.  It can be blocked easily by the rubber hose for filling or debris.  Both cases will make filling the thank difficult.  The first condition, blocked with the rubber hose, can be solved easily by having a less than square end on the fuel fill hose.  This will allow a tight fit over the nipple, and prevent the hose from seating all the way down so it will not block the vent.  Bubbles and fuel will come out around the fill when the tank is full.

If you suspect debris then carefully use a pin to carefully clean the hole.  This is better done with the engine taken apart.  Magnification may be needed. 

You can see light passing through the vent, adjacent to the fill nipple in these product images.
https://coxengines.ca/cox-.049-babe-bee-fuel-tank-backplate.html
better top view but the vent is less visible.
http://www.exmodelengines.com/product.php?productid=17652&cat=268&page=1
There are metal backs with this fill/vent arrangement.

The O-ring on the venturi is a bit difficult.  They can be made out of medium silicone fuel line by putting a short section onto a skewer and slicing rings off with a razor blade.
https://www.coxengineforum.com/t308-quick-and-easy-way-to-make-venturi-gaskets
although buying new is an option too.
https://coxengines.ca/back-end/
http://www.exmodelengines.com/home.php?cat=268

The seal around the back plate to the fuel tank bell is a little more difficult.  There are old recommendations of using floss or cord to provide a filling in the tank groove so the seal is better.  This does work but I gave up after a dozen to so tries and simply fill the groove in the aluminum tank with anaerobic sealant from the local car parts store.  I then clean the excess off and the inside of the tank.  When assembled this will provide an excellent seal against air.  I also use some of the same sealant on the venturi gasket and crankcase gasket. 

The last major problem is air leaks around the screw heads in the back plate.  A little more sealant _under_ the screw heads resolves this quite well. 

Finally if you suspect that fuel draw is still an issue, and you have cleaned or replaced the fuel draw spring (functions via capillary action) and fuel line  then look up the
Gibeault Mouse Race Program
and replace the pickup with a metal line as indicated.

Hope this helps!
Phil