Oh duh, indexing, of course! I couldn't figure out what you wanted the modern stuff for, forgot you were going to change the shift levers too. My bad!

I use both, friction and indexing on different bikes. I don't miss the indexing on the friction bikes, but I don't mind the indexing either, so I'm OK either way.

The R. der. hanger you need is common as dirt. You can use almost any brand other than Huret or Simplex. It doesn't matter if the hanger says Campy, Shimano, Suntour, Zeus or Gipemme, only matters that it has a threaded hole, and a notch for the rotation-stop at somewhere around 7 o'clock. Some hangers are longer, put the mech lower for a larger max cog size, but that's rarely a deal-breaker measurement, any length over a fairly broad range should work. Some (Campy) have 26 tpi threads where most have 1.0 mm threads, but that doesn't matter either, they're so close that either type of bolt will work with either kind of hole.

That hanger is sometimes called an adapter claw, so including the word claw in your search will get you more hits.

The original F. der that came on my SS is the only one I've used, but I have noticed over the years on other people's, that mechs with a steel clamp seem able to bend enough to fit the OS seat tube, just by using a longer bolt. Steel clamps are typically found on cheaper models. Mechs with alu clamp won't have the needed ductility to bend out, but might have enough metal to enlarge the hole, say by reaming, filing, Dremel or what-have-you. I wouldn't be surprised if there's a thread on it here somewhere.

Good luck and let us know how it comes out.
Mark B