Zastava made the 767cc and the 903cc engines so i can get as many of them as i want, about 16 years ago i build a stroker 1050cc from a 903did oversized valves bigger head bolts and that engine made some good power for its size but mid summer blow the lower radiator hose sized and throw a rod. They have only 3 main bearings and only one intake port. In my eyes they are the worst engine to make power. I'm planning to use a more modern renault K series engine when i rebuild my 600.