That plate and the rail normally come with the battery and the bit with the plug on can be moved to another plate. You have three pins, though it's probable that only two are used anyway. Unscrew the side panel and look in the compartment to see what wires are attached to the three pins. The voltage will be marked on the controller that sits in that compartment, and it might be written on the motor too.
It looks like a mount for a standard silver fish battery that you can get from anywhere in any capacity or voltage that you want. If you tell us the voltage and how much capacity you want, we can point to them.
You can see what you get here:
Silverfish Battery 36V 48V 10Ah 13AH 15Ah Li ion Lithium Electric Bicycle E-Bike | eBay UK