I have a four and a half deluxe sitting in my garage right now. I can spin the flywheel one way until it stops and I can spin it back the other way until it stops, it's got a broken rod and the crankshaft is coming around and smacking part of the rod that's not moving. If you lean the motor backwards and get the Piston to slide down then you can spin it all the way but the Pistons not moving so that's the problem. If you try to do a compression check it shoves the Piston back and re-locks up.
There really is no big mystery here. The fact you can't easily spend it with your hand with the plugs out tells me that Pistons are partially seized or the bearings or something or other so pretty sure your power head is fried.