Years ago my 550 got stuck in 2nd gear, lever still felt like it was attached to something but no way would it shift out of 2nd. I found the spring (#17 in the pic) was broken. The lever still had spring tension but the busted one stopped the drum from turning. I used an old distributor advance spring out of a car distributor to replace it. I don't know it this is your problem, but check it out when you get the clutch cover off.
Good luck, I hope you don't have to get further inside.