Reddit mentions of Tektro Caliper Brake for Beach Cruiser - 26" Wheel Front 79-99mm reach OE Packaging

The Tektro Beach Cruiser Caliper should work for you. Any shop in your area should be able to order them, and they are solid brakes.

I always believe that anything is safe provided you respect their limits. Coaster brakes can work fine and other times they can be disastrous... it depends on the rider.

But I also like efficiency and getting the most out of your equipment. It'll improve your performance knowing that you have decent brakes.

I like Tektro personally because they are good manufacture that represents very good value for money. You'll have to double check the measurements, but...

https://www.amazon.com/Tektro-Caliper-Brake-Beach-Cruiser/dp/B00BLM3TZO

Something like that. 20 bucks for the caliper, 7 for the cable, and 15 dollars for the lever. Little less then double of that for both front and back. A very nice upgrade for those is the salmon-colored kool stop pads.

Cutting cables and such things is a pain in the butt if you never have done it before, but a lot of videos online for how to do it.

You may want to look into caliper brakes as well. When I was building mine, the frame and hubs I had didn't have disc brake mounts, meaning I'd either have to weld them on or find really niche adapters (you may be in the same boat?). Most beach cruisers have caliper brake mounting holes though, it's usually just a hole sometimes filled with a bolt in the frame directly above the tire. This is assuming you don't have v-brake mounts which would be even easier.

If you look into caliper brakes, keep in mind that the front and rear brakes aren't the same, since their mounting bolts have to be different lengths.

I wonder if this bike will take brakes like these: https://www.amazon.com/gp/aw/d/B00BLM3TZO#Ask

If the front wheel you have has no disc mount holes you could just use the rim brake surface. This rim brake would mount on the hole of the fork.