Costco does not appear to be selling steel frame canopies anymore, only aluminum. I expect the current replacement parts would not be compatible in any way. Years ago, however, they used to sell steel frame ones under two different Item numbers - Item # 680344 that your dad gave you and the older Item #30354. The latter one appears to have been produced by Cozmo Products, Ltd. (1 800 929 9517) - and that company is LONG gone. 800 numbers are a hot commodity and they get recycled a lot, so someone else has control of it now - no point in calling it anymore.
We can still do some more investigative work here if you can send us the UPC by either posting a picture of the barcode or simply reading off and posting here the small numbers printed right underneath the barcode (be sure to get all 9 or 10). I am hoping that the original packaging might still be available and the barcode is still attached.
Also, can you post a picture of the manual page with the piece that's missing - need to know what that p/n 2004-A is. Since this question gets asked quite a bit, the more you can upload of the pictures of that manual the better. The form to respond to this question has an "image" button at the bottom that lets you post pictures along with the text.
Thanks!