...if you are replacing the front mech and the shifters, and you want everything to match, it's still pretty cheap and relatively easy to find one of the Shimano rear derailleurs that were made so that they would fit on Simplex rear dropouts without modification (other than threading the hole to the proper size and standard), and match it with a front derailleur and shifters from Shimano of the same era. Sometimes called "first generation Dura-Ace", the top of the line was "Crane". It comes in both long and short cage versions, and has a built in stop that works on Simplex. There were some slightly lesser models in terms of materials/finish that will also work. Look for "Titlist". Those are usually cheaper.
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^^^Simplex



^^^Crane


The short cage versions are cheaper to purchase, and have at least as much capacity as the standard Simplex. They are just more durable. The built in stop on the hanger bolt is of a similar design. I usually end up using a Suntour mechanism on the front, because they are just easier to find in the bins over at the bike co-op here. There are a lot of downtube and bar end shifters that will work. The Suntour ratcheting ones are very popular as a modification.
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