Yes, that's a "U brake". Other brake types really aren't compatible, due to the location of the brake bosses (above the rim, rather than below as for traditional cantilever or linear pull brakes). By your description, it does sound like you've lost some spring tension, perhaps due to the spring breaking. It's possible that a bike shop that specializes in older mountain bikes or BMX bikes might have a spring, but a complete replacement brake is probably worth the money (30 bucks).