That is a really cool chain ring. I, personally, would be happy to have a cottered crank with that chain ring on a 531 frame; cottered cranks are old school, but they're reliable, and the weight difference isn't as much as you'd think, especially if the arms are comparatively thin.

If you're set on going cotterless, though, I like Stronglight's suggestion you get a pair of TA or similar crank arms (Stronglight 49-d; there's also a Nervar version, something similar by SR or Sugino, and even Lambert / Viscount). With that, you can either (1) have a good machinist make you an adapter that bolts to the crank arm at the inside and threads to your chain ring at the outside, or (2) make such an adapter out of a chain ring that already fits onto the TA &c arms; you'll have to drill some holes through your lions, though, and if you don't do it really well, you'll end up with something like infinityeye's suggestion.

You know, it might be cheaper just to get a 94 BCD crank and have a custom chainring made for it, modeled on your old Peugeot ring.