Pictures would help to see if the parts are too rusted. I see in another post that you might be near Derek Tissinger. I would advise taking it to him if you are. He will also have used parts if needed.
I run 32:1 with a full synthetic 2 stroke oil. The 20:1 was based on lower quality oils. My biggest concern would be if the crank is rusted where the seals ride. That would tear up the seals quickly. It will suck air and blow up. If it started and ran, I am thinking the ring gear and flywheel are decent enough to continue using. As for used parts, the case halves are usually more rare than the other parts.