I presume by "frame" you mean the steel downtube that runs in front of the engine, and not the twin-spar aluminium main frame.
The engine is supported at four points, three on the downtube, and one on the main frame; once you remove the downtube, the engine will be supported only by the pivot shaft. There are two additional anchor points that will prevent the engine from rotating around the pivot shaft: the exhaust (which is rigidly connected to the frame), and the intake (not so rigidly connected to the frame). I wouldn't expect the engine to rotate, but supporting it with a jack in not a bad idea.