I have a Black Widow with about an hour on it that I'd like to resurect. Problem is the plastic back plate has two broken lugs. I understand that these were made with metal, and plastic backplates. Most folks offering new parts have plastic backplate. Question...Will the metal back plates from other Cox engines fit the BW? Or does anyone have a metal one the'd be willing to part with?

Ward

The old metal backplates fit the newer BWs fine (thank you, Tom)--also the production backplates, no tank of course.  You're better off with an external tank anyhow.

I have a boatload of backplates with lugs broken off.  One with the crankcase screws pulled through.

--Ray

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.