As per the “what will be in the game” thread I3 engines are the least number of cylinders planned.
As the tycoon mode is going to start post WWII (afaik) can I suggest that flat twin and single cylinder engines be included? Flat twin engines were very common in the 40’s and 50’s and have come up now and again since then. Single cylinder engines are probably less important but they have also made several appearances throughout automotive history.
Right top of the website offers a search engine. Search for two stroke or 2 cylinder and you will find topics addressing this question.
On topic: Possibly they will add these, but not after V6, I3 and other engines named in the “what will be in” topic. Maybe later, as DLC.