Hello, i’ve 2 quick answer to you developper.
4 and 6 cylinders flathead engines will be planned for the future ?
And in the future Maximum Bore & Stroke size will be increase to allowing 8/9/10/11 Liters Engines from the 40’s ?
Thanks you and you do very good job for your game 
Yoann.
Flatheads are not planned for this version of the game because they were already long in the tooth and on their way out in the 40s and 50s, it would also require completely new artwork and animations. There MIGHT be a DLC/Expansion in the future for past dates.
Also, not to be “mean” but please use the search function, this question has been asked many times over. 
I think is “NO”
It has been asked many times over.
Here are 5 threads that pop-up in the first 2 pages of results when entering “flathead” in the search bar.
viewtopic.php?f=15&t=6277&hilit=flathead
viewtopic.php?f=14&t=6122&p=68888&hilit=flathead#p68888
viewtopic.php?f=14&t=4204&p=37484&hilit=flathead#p37134
viewtopic.php?f=14&t=1308&hilit=Flathead#p12279
viewtopic.php?f=13&t=4069&p=36834&hilit=flathead#p36291
So next time, please use the search function.
Flatheads are easily in the top 5 of most suggested things, behind diesels, wankels and driving your own cars.