Well, spent an hour on the bike today and It's not looking good.
I removed the left side cover, and tried to manually turn over the engine using the rotor/rotor bolt. Everything inside the stator cover looked ok (I used this thread "
Stator Clutch Renovation" as a reference).
After turning the rotor nearly one full revolution, I found it hit resistance. I didn't continue because I didn't want to force it. That made me think – it could be the pistons are jammed somehow.
I chose the least invasive route, and removed the spark plugs and used a little flashlight to see what was going on. On the right side, nothing interesting was going on, other than fairly white spark plugs.
But the left spark plug… well I'll just leave this picture here. And a picture of the view of the inside of the cylinder through the spark plug hole. Pretty, pretty, pretty, pretty bad.
Also, here's a little video of what the inside of the chamber looks like (sorry for the length – start near 25 seconds in).
Advice on what to do next? I haven't seen this issue in my search on this forum. I do have a second engine so I can salvage parts from it…