Continental engines were used in the M35 2 1/2 ton Army truck, in the early 50's. The installation could have been done in the mid to late 50's with a surplus engine sold by the Army. They were also used in Reo Gold Comet trucks, one of which may have migrated to Germany after the war.
I saw this, and the engine looks very similar to yours.
https://www.easternsurplus.net/PartDetails/4941/REO-Gold-Comet-Continental-Model-0A331-Gas-Engine