Well, this is difficult. http://manuals.monstermotorscooter.com/XB-600-explode-diagram.pdf Sorry, but YOU have the bike, you have to provide us with dimensions.
Might this be right? https://electricscooterparts.com/drumbrakes.html If you just want to try something 3rd party, you might have to improvise a little. I'll say don't mess around much when it comes to brakes. I remember college and trying to reline the brakes when someone was broke. The results were unimpressive.
The X-Treme website has it, but yeah, pricey. http://www.x-tremescooters.net/products/parts/Mjk=/xb600-electric-bicycle.php
So, I'll let you in on a worst kept secret: So many of these Chinese parts are KNOCKOFFS. Common on scooters are the Honda brakes for the CRF50/XR50. Look familiar? http://scooterpartsunlimited.com/dirt-pit-bike-front-rear-wheel-brake-shoe-drum-hub-for-50cc-honda-crf50-xr50/ Or they may have been a knockoff of something else.
But you'll need to figure this out for yourself, I'm not finding a reference to it.