Yeah, the Simplex front derailleur braze-on is incompatible with anything else. You can find the derailleurs on eBay every so often. Maybe try ebay.fr to get closer to the source?
The good news is that if you find one, they're actually a good performing derailleur.
If you choose your chainring sizes carefully, you may be able to mount a Campagnolo front derailleur above or below the braze-on, but chances are, it's going to be in the way for common chainring sizes.