WD40 won't help. You need the real stuff.
Your options are not great if you strip it. At least on an SL there is no room to get a cold chisel on it. Anything involving a drill or Dremel tool would blow because of the metal shavings.
Maybe others have suggestions, but if it happened in my garage. I would probably end up pulling the head.
I still think if you are taking it out to change the lining, you should look into changing it in place.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe