I maybe making it up but is one of the main things with a false 9 that you don't need a number 10 as much because he is kind of doing both a 9 & 10s job? Messi was that, Firmino was that and even in the recent City games I think I saw Foden there or something.
Generally speaking, it's very hard to play a False 9 and a true #10 together mainly because they end up occupying the same space on the pitch and ruin the structure of the attack. If you have Bruno playing behind Martial, it's not quite as problematic because when Martial drops, Bruno charges into the box so the spacing isn't as much of an issue. But in general, I'd say most teams who play a False 9 don't play a true #10.
In terms of finding an actual False 9, that would be very difficult. Ajax used Tadic as a False 9 successfully but that involved shifting him from out wide. Out-and-out False 9s aren't really that commonplace as most teams tend to prefer more traditional CFs so finding a good False 9 will be very difficult. All this kind of makes the issue of replacing Martial very difficult because unless you're willing to not build your attack around Rashford on the left and Greenwood on the right, you'll end up having a surplus of finishers in the team and a deficit of creators, with the spacing being badly impacted as well.