This is for a RC truck/car? If so, what vehicle? There are much better engines than the long discontinued CZ engines.

This thread should be moved to the RC Car Engines forum for one. For two - any modern .12-.20 smallblock engine will work just fine and will be a much better engine than the OS CZ engines. Personally, I’d go with a Novarossi .15 myself. However those are rear exhaust. If your engines are side exhaust (which I suspect to be the case), you will be far more limited on options. Modifying the trucks to suit a rear exhaust engine would be my suggestion (as well as probably changing the throttle linkages to allow the use of a slide carb).

Well, whatever you decide to do, I wish you the best of luck.