I'm sure it's possible something fell in there - I was really careful, but who knows.
After I took the piston off I could rotate it a full 360. What's the next step on finding out what it is? I couldn't figure out how to compeletely open the engine, I only got the side off with the gears inside.
Also - I tried removing the flywheel off the magneto by unscrewing the big nut - but it wouldn't come off. I couldn't find anything that was actually holding it on there. It will rotate like 1cm in both directions, but nothing more.