Have you considered swapping out the freewheel (and possibbly rear derailer)? I don't know if your rear spacing would permit it, but you may be able to find a freewheel with a bailout gear for those hills.

I understand that you like the looks of your cottered crankset, but you might also be able to find a used cotterless crankset and bottom bracket that looks "right" on your bike. If nothing else, see if your LBS has anything in the backroom. Cottered or cotterless, it might be easier to cast a wide net and look into a different crankset, instead of seeking only chainrings for your current crank.