The Workbee does use c-beam, for all 3 axes, which is a lot stiffer. The plates make no difference.
With the Ox design, at least the Ooznest design, there is really no way to use C-beam for the Y rails because of the inner plates. You could theoretically use double rows of wheels, but you’d still be stuck with belt drive, as the plates have no provision for lead screws.
No, C-beam cannot be used on the Ooznest X axis because there is no way to mount it, it is 20mm wider than the 60mm Ox X axis. Also, the wheel spacing on the X plates is set up for 60mm width, and an 80mm C-beam won’t fit.
The Workbee is a far better machine. For the $50 you spent on those plates, you would be much better off to just scrap your Ooznest Ox plates and buy a Workbee, than to try and modify the Ox. You would still be stuck with belt drive.
I have bought and assembled dozens of the kits from Bulk-Man 3D, (the Ali Express dealer you mentioned above) they are very good quality, easy to assemble, and cheap. Even with Fedex shipping they are cheaper than OB. I would definitely not go larger than 1000x1000mm, though, because it is belt drive, and the stiffness really suffers when you get longer than 1000mm on any axis.
The Ox design is a bit long in the tooth, and the Worbee class machines are much better, IMHO.