The Mylar reed which is the same shape as the original copper berylium works quite well not too mention is readily available. Once the circlip is in place, I like to rotate it in order to insure it's seated. When I'm satisfied with that, I take a X-acto and carefully nudge the reed underneath in rotation to verify it's not pinched. I then hook a syringe to the venturi and do the suck blow test. Don't just pull off the old hard pick up tubing from the backplate. The fuel nipple on the Pee Wee is very fragile and can break off. Use heat first and this will make the tubing flexible and it will slip right off. Wiggling the old one prior to what I mentioned will immediately snap the barb off.
One important thing I like to verify is the area where the venturi of the tank plugs into the backplate. Inside this area, there's a small hole. This is where the point of the needle is located. Remove the needle and insert a fine piece of wire down through the spraybar and insure it exits the hole I mention. Repeatedly do this as build of crud isn't uncommon here and alcohol can help break up the crap in there. Prior to reinstalling the fuel pickup tubing, insert a modeling pin back through the fuel barb with the needle removed and insure that you can see the pin through the spraybar hole looking down into it.
When doing the suck blow test, a drop of oil is sometimes needed on the reed which will assist the reed in sealing. If I find it's leaking in anyway, I always flip it over before I try anything else. When the Pee Wee is back together, I pressurize the tank to insure no leaks as well. If air is immediately escaping, check the screws on the backplate, listen at the venturi intake as that is a immediate sign the venturi o-ring isn't sealed. Place a drop of oil on the threads at the top of the backplate where the needle is inserted, this can yield air leaks where the spraybar is pressed into the backplate and also threads themselves. Keep in mind that other leaks can also show their ugliness such as tank to crankcase, screws through the backplate and tank to backplate connection. While some of these might have a moderate impact on engine run, all of them at some point matter and should be resolved.