It looks like one of the old engines made in Eastern Europe before the iron curtain fell. It may be impossible to identify it, as the machinists at the time were very reluctant to be linked to the model engines. There were a number of people that made engine over there in quite a few different countries too. But very little information as to who made what.

I cannot tell from the picture as to the displacement size of the engine. But MVVS during their days behind the iron curtain did make some 10cc engines that look similar to it. They had a lot of varieties and differences between the versions too.