+1 Cheapest option is a claw type hanger for the rear derailleur. Its really not that bad of an option, many older bikes used this style derailleur hanger, so it is period correct, just not model correct. Otherwise, have a frame builder replace both drop outs. Thats a pretty expensive option, will be nicer, but will it be that much nicer?

You will see a variety of vintage rebuilds on the C & V forum with claw hangers. That's what I would do. You will need a longer axle, which is not a big deal.