Unfortunately many people in the hobby have no idea on engines, because of lack of experience. Your OS sounds like it may be stuck due to the oil setting up over time. About the worst thing you can do is attempt to turn over a gummy engine, because the bearing's balls will slide instead of rotate. Doing this puts flat spots on the balls and degrades the bearings. The best way to loosen the engine is to heat it up in your oven to about 250-275 degrees. This temperature will allow you to turn any engine where it is only congealed oil. Add more oil to the bearings to help flush out the old oil while hot. I would take off the backplate and carb, while using a wood prop. Gloves are also mandatory.

Now most people think you are home free at this point, but they overlook another important area in the engine. The wrist pin may be frozen in the rod, thus the pin rotates in the piston. This is very bad, since the pin should not turn in the piston, but rather the pin should allow the rod to rotate back and forth on the pin.

Marginal bearings can sometimes be salvaged if you have an ultra-sonic cleaner. But to do this you really need to remove them from the engine case. This means you have removed the sleeve, the piston and rod, and the crankshaft. To remove the bearings, heat the case in the oven again, and slap the case down on a block of wood against the surface where the backplate mounts and the rear bearing will just drop out. Avoid using a torch, as it is difficult to evenly heat the case and you can damage the case. The front bearing will push out with the crankshaft from behind.

Use a pencil to mark the rod before you remove it from the engine and leave the rod in the piston unless it is frozen to the wrist pin. Pay particular attention to how the wrist pin is held in. There are three or four common methods of assembly, but circlips are most often found. When ever possible, use a new clip or pair if you remove both, and pay attention to the direction the wrist pin goes in. It takes a very very small set of needle nose pliers to remove circlips, and is also needed to put them back into the groove. You will also want to use a #11 exacto to make sure the clip is in the groove. When working with piston rings or circlips be sure to wear eye protection.

By no means does this cover everything you need to know to take apart engines or to reassemble them. That would take a lot more time to write. Bear in mind, that many engines have thin shims or special parts that you may not notice when taking apart, unless you are paying very close attention. Lose one or leave it out and it will not turn over when reassembled. If the circlip breaks or comes out, you will lose the piston and sleeve. If the main bearing breaks down, you will damage the piston, sleeve and even head. So the condition of bearings is important. To test the bearing when out of the engine, hold the outer race and turn the inner race with your index finger. It should be smooth when pressure is applied in the radial direction (like as if the prop was pulling against the bearing).