All the "modern" reed valve Cox engine parts (at least Babe Bee on) are completely interchangeable. Just need to watch out for where the fill holes are on the tank or backplate. I think what you are asking is the production backplate which also includes the reed valve and venturi. Yes if you take off the integral tank, you can use this backplate and an external tank. The only thing to watch for is that the total package is now shorter (I know that is obvious) so the balance of a plane setup for the integral tank version probably would be tail heavy.
Also as someone mentioned, the venturi may be smaller diameter on some of the integral tanks. Larry Renger posted how to hop up a Cox Black Widow (I think) over on SSW, and gives more details like venturi diameters.