M3s and MP-38/40s were sold as CAs, even though they were never semi auto. The Sten tripping lever has a notch which presses against a ledge on the sear, and the bolt strikes a trip on the tripping lever to disconnect. In the others, the trigger connector is pinned to the sear. Turn this into a movable connection like the Sten, and add a projection to the connector for the bolt to strike, and its semi. A spring to keep the connector in contact with the sear would be needed - the Sten system is as easy a way to do this as any.
The M3A1 is as simple as they come. But its method of construction was more complicated than many.
Really, you need to decide if you want a close reproduction, or just a semi auto pistol calibre carbine.