It is a common problem on any machine that has thrown the chain. Check the shifter shaft for marks on the top that look like the chain has gouged into it, also check to see if the shaft is bent. I would recommend getting that crack welded up, if the part comes off completely, you will lose all your oil and I'm sure I don't need to tell you why that would be bad. Also, a good case saver will prevent things like that, and are a must on any chain driven machine IMO.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.