Is there a way by which I can say activiti engine to execute custom script engine rather than the one used in actviti.
My all tasks uses async behavior, so lets say when I execute a script task workflow then instead of 'ScriptTaskActivityBehavior' it calls my own custom handler?. I am using activiti-engine-5.13.
Yes sure. You can use your own Java service task of course. If you want to use the script task BPMN element you can use your own ScriptTaskParseHandler and overwrite the ActivityBehavior class executed when encountering a script task element.