Hi PD. I would just go grab a universal foam air filter from your local bike shop. You CAN buy a replacement pleated regular air filter but if I was going to ride it I would put a UNI type foam filter on it. They work much better and just about every motorcycle dealership stocks a display of universal foam filters. I have the original filter in my 160.
They are pretty big and would have to be super dirty to restrict the air flow enough to hold your engine back. I road my CA95 around without an air filter for a while. Just riding around on the street won't put a whole lot of dirt in your engine considering the location of the air intake points inside the frame.