Mark!!
(Photo from Walther's website)
The bracket really isn't anything special. It just needs to clear the workings under the turntable and the motor has to be in perfect alignment with the bridge. It is also (IIRC) glued to the bottom of the turntable pit. Just mount up the motor to the bridge, and measure for what is needed to fasten it in place. I'm going to give you fair warning however, getting this thing to work properly is an exercise in patience and sometimes, futility.
Good luck..
Try sending an e-mail to Walthers. They will certainly sell you one for a nominal fee, and my guess is they're just as likely to send it to you for nothing, if they have one in stock. Let them know you bought the kit second-hand, and when you opened it you found the part was missing.
They enclose a note in some of their kits that says "We want you to succeed." They even say they'll replace a part if you break it during assembly.
It takes an iron man to play with a toy iron horse.