The easiest solution would be to buy a mountain bike with a disc front end. You'll also need a 1" steerer fork for the cruiser frame with a disc tab if the mtb is not very old (most have 1.125" steelers). The components of a disc system are fork, brake caliper/lever, disc hub, disc, and caliper adapter. The caliper adapter offsets the caliper based on disc size and fork mount type. I.e. If you buy a suntour suspension fork with 1" steerer and post mount disc mount, and a disc system with a 180mm disc, you'll need a post mount 180mm disc adapter.
Also since this is a motorized bike, look for disc hubs with bolt on axles (not quick release) although a qr can be okay.