The stuck valves like everyone has mentioned is a very common problem on 4-strokes that have sat a while. Carbon and/or castor oil residue gets between valve stem and the guide and it sticks in whatever position it is last left in. I've bought several used engines that people could not get running and found this to be the problem. This is why I also like to store them with the piston at TDC of the compression stroke so both valves are closed.

I've also seen a few bent pushrods on used 4-stroke engines. My guess is that a valve is stuck and the pushrod ends up bending because of it when the engine is turned over.

If you do have a stuck valve, a little fresh fuel might break up the gunk or good penetrating oil will often do the trick. In extreme cases you may have to disassemble it and clean the stem & guide but that's rare.