Well, IOTL I know that Christian minorities of up to 20% (in Algeria specifically) existed during the colonial period, although these were largely the result of European immigration to the region. These could be kept perhaps with a less violent decolonization (maybe France doesn't get crushed during WW2 and can devote more resources to the colonies), or even perhaps by just having the colonies survive as such.
Native conversions will require a change in the colonial process. France was the dominant power in Morocco, Algeria, and Tunisia, and they tended to prohibit attempts to convert the locals. With a French government more interested in spreading Catholicism (maybe restored monarchy does better) we could at least see attempts to convert the people by the church, and the government looking the other way. Whether conversion attempts would be successful I have no idea, but there will always be people who convert to make the colonial authorities like them, so I'd say there is a fair chance at significant conversion.