Your fork doesn't appear to have disc brake tabs.
Your frame doesn't appear to have disc brake tabs.
Your wheels doesn't appear to have disc brake hubs.
With a $ 160 price tag on the complete bike, and all the parts you'd need to buy, I'm struggling to see a sensible way forward to achieve what you want.
Why not start by staying with rim brakes, and work your way through modest upgrades?
-Salmon pads
-Well routed, high quality cables
-Maybe some brand levers and v-brakes. I like Avid Speed Dial and their Single Digit brakes. But Shimano XT have a good rep as well.

Heck, even something like the Magura hydraulic rim brake Rim Brakes*| MAGURA might be one of the least silly ways out for you.

Getting a rear disc brake might not be too hard, if you can get something like this to work: Robot Check

But why a motorized bike?
Apart from the possibility of pedalling in case of running out of fuel, I don't see the point.
Wanna go brrm, brrm, get a scooter or a moped.
Less risk of flats, less maintenance, built for the task.