I don't know if you can buy just the top plate or not.
The battery tray is not too long. The mounting holes in the tray don't line up with the motor mount holes in the chassis.
The MBX-6 has the motor moved forward a good bit and in towards the center a couple of mm.
If your in a hurry. By Dan's motor mount and top plate and get an R/C Monster battery tray.
I believe Dan will be working on a tray soon.