The engine could still be good, and not need a rebuild, if it was in good condition when it was used before.
When you get it to turn, do a compression test on it and see what it has, then you know what the compression is.
It could have stuck valves too, they need freeing if there is any.
The rings could be stuck on the pistons, and need freeing. The guys on here recommend the oil additive that they said above. It takes some time running the engine for it to help.
If the engine hasn't been run in several years, you should prime the oil pump, they sometimes lose their prime and won't suck the oil up from the oil pan. Then there is no oil pressure when the engine is running.
Below is a picture of where the hole is that I use for priming the oil pump. Just remove the oil filter cover and oil filter, and it is easy to see, use a light if you need to.
I fill a clean oil can with motor oil, and pump it into the hole, you might need more than one filling of the oil can to fill the passages in the engine.
Watch the oil gauge after starting, it takes 15 - 20 seconds for the gauge to show pressure, it has to fill the oil filter housing before the gauge will show pressure.
Great advice above on priming the oil pump using the small hole inside the oil filter area. Just one caution there: after you prime it as Glen mentioned, make sure you put the filter and cap back on before starting the engine. If you don't you will be wearing about a pint of oil on your shirt and in the area where the Cub is sitting. I know, because that is what I did the first time! The oil came out very quickly and under pressure.