If you can easily turn the flywheel by hand with the plugs out, I sincerely doubt the problem is something that is seized. Follow Jerry's lead on doing compression tests and then post your results. If readings are within 10% of each other with a range of 90~150 PSI, then move on the next step of troubleshooting. But if difference is high between cylinders. the next step is to open and inspect by removing the head. Try saving the head gasket if possible.
Disconnect the large black negative wire on both ends, i.e. battery and engine block. Wire brush it until shiny and the same with the mating surfaces, i.e. battery negative terminal and engine block. Use dielectric grease to coat both mating surfaces and reconnect tight. Next do the same with the large red battery cable. There are two sections of this large cable with the solenoid in between.
After reconnecting, test the connections to make sure they are tight by bypassing the solenoid. Using a medium length screw driver short the two large terminals on the solenoid. If the starter cranks fast and normal then you know all your connections are clean and tight. Now try starting using the ignition sw, if the starter cranks fast and starts the motor, your are basically done. Hence the problem to start with is dirty or loose cable connections. But if the starter cranks slow when using the ignition sw, then the problem is most likely a worn out solenoid and needs to be replaced.
If after shorting the solenoid terminals and the starter still cranks slow, then dirty or loose cable connections is not your problem. Now it is time to check the starter. Make sure you are using a fully charge battery, using battery jumper connect one end to the starter positive connections and then the other to the battery positive terminal. The starter should crank normally if not the starter is probably in need of cleaning or replacement. But before doing so, do another diagnosis. Remove all top three plugs and do a quick start, if starter cranks fast, install another plug on #3. Start again and continue to do this until the starter starts to indicate its problem of dragging. When this happens take note of the cylinder you last installed. There is possibility you might have some ring problems with it.
However, if the starter still drags with only one plug in, move the plug to another cylinder. If the starter is still dragging, then the starter needs to be cleaned or replaced.
