It's hard to tell from here what's wrong with the engines.

Sport pilot is right that some oil residue can look like rust. But that doesn't mean you haven't rusted your engines either.

Engines with rusty bearings don't turn smoothly, there is resistance not related to compression, and the movement often feels jerky or "crunchy". If you are getting that kind of feel, you can repair the engine, but it may or may not be worth while depending on how bad it is. Often all you need to do is replace the bearings and maybe the crankshaft.

The OS .50, is it ringed or ABC? If it has a ring, then low compression when cold isn't necessarily the end of the world. However if rust is a problem the ring could have rusted, and the whole engine could be junk. If you flip it backwards, do you get better compression? For ringed engines, you often get better compression going backwards, and that's a good thing. (flipping backwards actually more closely simulates what is going on inside a ringed engine when running).

Your best bet is to find someone near you that has experience taking engines apart and fixing them.

If the engines have rusted, (and it sounds like they probibly did), then what britbrat said about preventing it in the future is correct. In the future, when you are done for the day, empty the tank, and then start the engine to quickly burn out any fuel left in the engine. Then put in a good shot of oil and flip the engine a few times to get the oil to travel all the way through the engine. It will go a long way towards keeping your engines from rusting up.