Originally Posted by 3alarmer

...the simplest approach to changing out the rear derailleur on a bike with Simplex (presuming you don't want to pay the exhorbitant prices asked for the Simplex used long cage ones on ebay), is to go with a Shimano Crane or a Shimano Titlist in the long cage versions. They both sork about the same, but the Crane has slightly better finish, and is lighter in weight. They have an integral stop washer that will grab the Simplex dropout on the edges. Then you just need to enlarge the 9mm hole to10mm, and therad it with a 10mm fine thread tap. then it bolts right in. But your bike is new enough that it might be originally made with a Campagnolo style dropout. I can't tell from your pictures.

I still would not do this project, because it's starting to sound like some real money in parts.

Those Stronglight cranks are worth money to someone else in the C+V world, who is more interested in originality than you. It's considered a nicer French crank.