Disconnect the clutch electrical connector, run the engine and turn on the PTO and see what happens. With the engine shut off, with a multimeter in the Ohms setting, check the resistance in the clutch coil. Also check clutch terminal to ground. Let us know what your readings are. Clutch should be no less than 2.6 Ohms and infinity from each terminal to ground.
Well it would be helpful to know what engine we are talking about, model number. IF it is a twin cylinder, is it running on both cylinders? Sounds like it isn't to me. IF it is a twin cylinder B&S in particular, remove the valve covers and check the push rods, etc.