Goo in the crank...
I have used several types of filler on motorcycle cyl's and heads. I have also used some on R/C engines. Try Belzona (very expensive!!) or Devcon Aluminum Putty (part # 10610, about 30 bucks), I have ad good experience with both.
Just don't try and put it in the exhaust port, it won't live there long! Intake and crankcases only!