There are so many problems that can occur with these engines and a few issues have been touched on. Air leaks are the culprit of the problems. Air leaks develop around the screws and from what you mentioned, you've already distorted the back plate. Obtain a large syringe preferably a 5 oz., screw the needle valve down until it's seated but not tight enough to ruin it. Pressurize the fuel tank with the syringe. Listen carefully for air leaks. Use oil around the perimeter of the backplate to tank connection. Place oil on the crankcase to tank connection. If it's bubbling, you have found some leaks. With the backplate facing up so you can see the screwheads holding the tank back on, place oil on all 4 screws and pressurize again. You will see bubbles around the screw heads, there's another leak. Check the needle valve assembly as well with oil on the top of the backplate while pressurizing.
You can lightly sand the crankcase back on a piece of glass using 400-600 grit with a little oil. When screwed together, the crankcase is extruded and the material tends to pull out raising a bump around the threads. Flattening this assists in sealing. Insure using a 2-56 bottom tap that your crankcase threads are chased and clean.
You can fill the v-groove inside the perimeter of the tank with a piece of dental floss as a gasket holding it in place with some oil prior to assembly.
Backplate screws are fillister head screws and they tend to wear out. You can obtain new stainless ones from Matt at EX Model engines or Cox International. Replacing screws can sometimes resolve leaks but I use a Q-tip. I remove the screw and tear off a little piece of Q-tip and twist it up and wrap it around the screw threads the same way you would use Teflon tape. Insure that when tightening, the cotton tightens and doesn't unwrap. This works great and is serviceable at the field if needed unlike silicone or Permatex.
Lat but not least, when pressurizing the tank with the needle closed, if you place oil on the screen and it's bubbling when pressurizing, the O-ring is leaking. Cox O-rings totally BLOW!!. Use a real O-rings for throwing darts which you can obtain easily on EBAY or sporting good store. Yes some people can make them from fuel tubing but I've found the dart O-rings to be a very good fit over anything else I ever used.
While this may not be a issue, ALWAYS, when the engine is disassembled, hook your syringe to the back of the venturi on the tank. Regardless if your reed is new or not, withdraw the syringe. You shouldn't be able to pull the syringe back AT ALL. You can push it forward but it shouldn't withdraw. If it even slowly withdraws, the reed isn't sealing properly. Take it out, clean everything, flip it over and try again with a little oil on it. If it still fails the suck blow test throw it out and try another. The reed must be able to rotate under the circlip. The old copper beryllium reeds can be damaged so easy you need to be extremely careful as the slightest indent can cause it to fail. My preference is the clear Mylar reed but they have a short lifespan, however, they can be extended by flipping them over but they offer terrific performance and tend not to gum up as badly as the copper reeds which gets the green goo under them.