I'm betting it's the switch and something has jammed it in the "ON" position. Key won't come out, of course. So I'm betting it will be easier to just remove the switch as a unit, then disassemble, clean and replace if necessary.
Duck, looking at those security bolts in the parts diagram provided by Inge, it appears that they used LOCKTITE, to secure them. Did you ever try heating the bolts rather than drilling?