If I was trying to find out as to what is going on I would segregate the system, meaning removing the chain. By doing this we break the system in three areas. Now you can work on one at a time. Rear wheel first..does the wheel turn easily and freely?...no...there is/or part of the problem...yes?? Now to the engine. Remove clutch cable from clutch arm. Remove spark plug as stated, remove the magnetto cover, using a socket rotate the magnet shaft (crankshaft) clockwise, do NOT use excessive force as you can strip the nut. If need be remove the clutch cover, remove small plillips screw, and back off on the star nut (Clutch adjusting nut) relieving all clutch pad contact and try to turn engine over again with socket. If need be remove the clutch pressure plate and check again. Make sure that the magnet is not binding with the magneto coil. If both rear wheel and engine turn freely you have a chain/drive, or a clutch cable/linkage problem. All this if done by a knowledgeable mechanic should take no longer then 1/2 hour from start to finish. Tack on additional time if you are unexperienced.