We can get the Nankang CX-668 through America's Tire. They seem to be well regarded in the VW Beetle community, and a couple people on BMW2002FAQ said they're decent. The tire has a H speed rating, which is unusual for an all-season at this price point (and relevant because my car was originally equipped with H rated tires). So for all of those reasons I ordered a set from America's Tire. America's Tire says they only have five (!?) left in their distribution network, so hopefully they make it to the install on Monday. My first choice was the Toyo Extensa A/S, but my online order was cancelled because only 2 were actually available (from America's Tire, at least), and I'm told the model will be discontinued soon. So looks like the pickings are getting slimmer.
The Altimax RT43 comes in a 175/70r13. I seriously considered moving down to the smaller size because its well regarded, but the cons would be a too low of a load rating (82 vs 86), a smaller contact patch of course, and a slight "roller skate" look to the car.
But good on you for getting the CN36, FordCapriDrive! In the BMW 2002 community, there's a contingent of serious enthusiasts that say in their eyes there is no substitute. But I just got the car and need new tires to actually start forming an impression of it. If I decide to get serious about E21s I might justify spending $700+ on tires, but at this point, while my example is not falling apart, it is on the rougher side and tires of that caliber wouldn't be on par with my investment in the car generally.