An arch is not a truss.
And do you see those zigzag pieces that make those arches into arched trusses? They're straight. They wouldn't work nearly as well if they were curved. The arches themselves would work better as trusses, like for like, if they were broken up into straight segments, like a geodesic sphere:
Like it or not, this is relevant to the OP's question. "I've got a cheap curvy cruiser with terrible wheels and minimal brakes. How do I beef it up?" The best way to add strength to that bike is to substitute a diamond frame and good aluminum rimmed wheels. But that means selling it and getting something intrinsically stronger.