Hi, I'd like to try and assist you in locating this part! If I am understanding correctly, you are missing the plastic back that the engine ( part #900503) is affixed to, and let me guess, without it you cannot mount the coverplate? Is this a recently purchased item? The engine should have this part already attached, and purchasing a new engine seems to be the only way you might be able to get one. If this unit is new, have you re-checked your packing materials? As I am unfamiliar with how Gainsborough Stanza packages their product, some manufacturers often have these delicate components packaged in the styrofoam that secures their product in the packaging. I hope this helps you! Please reply if I can be of any further assistance. By the way, if you do find yourself in a position to have to order this and cannot get it without also having to get a new engine, I would first check ebay. There are some units I've notice on there for parts, and you may find just what you need there for a much more reasonable price.