That is one solution. There are 3 solutions that I can think of.
1. Purchase the cable mentioned above.
2. Rewire the 3rd party cable so that pin 7(console end) connects to pin 20 (scart end)
3. Connect a wire from pin 11 (cxa1145) to pin 3 (AV port). A wire should already be soldered from pin 11 (cxa1145) to pin 7 (AV port) on the French mega drive so the wire can simply be moved.
It all depends on what the op feels comfortable with.