I you're going to put an engine on it then yes, you need additional braking. I got my last 4 parts donor bikes at Goodwill. $7 apiece. 2 had usable caliper brakes. 2 had usable MTB suspension forks with braze-ons and V brakes front and rear.
If you don't want to swap forks, most cruiser forks have a hole at the bottom of the steerer for caliper brakes and/or fender. Most also have a brake bridge at the top of the seat stay for mounting a fender and/or caliper brakes.
I would definitely put additional brakes on it. I would add front calipers, at the very least. Coaster brakes are not dependable enough on their own for the speeds of a motorized bike.